You're asking one of the hardest questions in the industry. Tabletop game publishers are very reluctant to release firm figures, being very skittish about appearing as 'weak'; this problem has plagued the industry since I got in in the early 90s, and doesn't appear to be going away anytime soon. Also, the numbers you report are self-reported, which is not ideal. However, we can infer that it's a tiny subniche of a niche market from several different sources:

First, ICv2, the trade magazine of the industry, has reported the total volume of trade done in the RPG industry is $25 million in 2015. (This is actually an increase of $10 million over 2014.) No matter how good an OSR game does, this is the upper cap.

Second, no 'indie' game is in the top five.

So, if they're not breaking ground in the major games market, let's check RPGNow (another brand of DriveThruRPG); OSR games like Labyrinth Lord have made major headway there. Sorting the OSR category by popularity, we see that the best seller for a game tagged as OSR is OSRIC Unearthed, which RPGNow marks as an Electrum Seller. As an Electrum seller, it has sold at least 250 copies, but not 500 copies (to make Gold).

Finally, let's check their top 100, which scales games over time. The highest-ranking game is Guardians, #29 in the Hot 100. What does that mean? Well, this blogpost by 6d6 Games reveals that their game, which reached the position of #13 in RPGNow's Top 100, made it up to 38 sales. The numbers are not directly comparable (since it's sales-over-time, and the figures are from different years), but it illustrates the size of the OSR market.

That said, if you want to make your OSR games, there is an alternative - crowdfunding. The most successful OSR game on Kickstarter right now is The Black Hack, which reached 604 backers.

So no matter which way you cut it, us old-school murderhobos are a rareified breed.

General

• Stat Generation: Pathfinder Second Edition (PF2e) has moved away from the methods of the past wherein you generated stats using a point buy or rolled them. Instead you receive various ability boosts (a +2 for a stat < 18 or a +1 if the stat is >= 18) from differing aspects of your character that you decide upon over a series of steps. Additionally, you receive 4 additional ability boosts at level 5, and then again at levels 10, 15, and 20; these boosts are applied to 4 different stats.
• New XP System: Instead of having a table for the various XP needed to have and gain a level, you always require 1000 XP to go up a level. When you do so, you subtract that 1000 XP and need to gain 1000 XP to level up again.
• Proficiency: Instead of just being proficient or non-proficient with something (or trained or untrained as was the case of skills), you now have varying degrees of proficiency: Untrained, Trained, Expert, Master, and Legendary. If you are untrained in a thing, you have a proficiency modifier of +0. For trained and above, you receive a modifier of your level + 2,4,6, and 8, respectively, depending on your degree of proficiency.
• Keywords and Traits: Almost everything has a keyword, these keywords have specific definitions for what they mean, and how they apply. Additionally, almost everything has a trait; these traits extend from category markers, to affecting how a spell works.
• Multiclassing: Multiclassing is now completely different. Instead of selecting a class level, you instead take an archetype dedication feat in place of a class feat (see below) and gain the ability to take other archetype feats with that dedication feat as a prerequisite.
• Critical System: You now have 4 degrees of success on a check: Critical Failure, Failure, Success, Critical Success. If you roll 10 or more above the DC of a check, you critically succeed on that check. If you roll 10 or more below the DC of a check, you critically fail. A natural 20 on the die roll moves you up one degree of success, and a natural 1 moves you down 1 degree of success.
• Modes of Play: Events in the game are now divided into 3 different modes of play, to represent the difference in situations, stakes, and their time scales. There are various abilities that may work differently (or not at all) depending on the mode.
  • Encounter: This is the mode of the gameplay that combat takes place in, and is thus the most structured one. Not much has changed aside from the action economy in relation.
  • Exploration: This is the mode used for wandering dungeons or the dark forest, where there may be dangers lurking, but none are immediate. This is typically spent in 10 minute increments for a round of actions.
  • Downtime: This is the mode used for crafting, earning money, treating diseases, etc. The smallest unit of time frame in this mode is 1 day, and should there be dangers or other things happening, Encounter or Exploration mode should be used instead.
• Action Economy: In encounter mode (see above), you gain 3 actions and a reaction each round. Actions can be used on your turn, some abilities take a single action, while others may take more. Reactions can by used at anytime the trigger for them are met. Free actions still exist as well.
• Perception: No longer a skill, it now also encompasses the various senses of a creature and how detected a thing is.
  • There are 4 different stages of detection: Unnoticed, Undetected, Hidden, Observed.
    • Unnoticed: If something is unnoticed by you, you have no idea it is present. Like the nanobots monitoring you.
    • Undetected: If something is undetected by you, you know it exists in your general area, but you don't know where. This is the set of car keys that you set down somewhere in the room, but don't remember where.

      They're in your hand!

    • Hidden: You know the space the thing is in, but you don't know where exactly it is in that space. This is the remote that fell into the couch that you can't find.
    • Observed: You see it, it may see you. This is the answer you're currently looking at and up-voting.
  • Senses fall into 3 different categories: Precise, Imprecise, and Vague.
    • Precise: Essentially, these senses allow you to make something Observed by you. For example, your eyes are observing this text.
    • Imprecise: These senses allow you to make something Hidden at best, but not Observed. For example, your hearing allows you to locate where that funky whirring sound is coming from, but not what it's coming from.
    • Vague: These senses allow you to make something Undetected at best. For example, your sense of smell makes you pretty sure your roommate exists and needs a shower, but you can't tell where he is nor do you want to get closer.
• Bulk: Instead of using weight as the unit of measurement for carrying capacity, you now use bulk. Each item has a Bulk of a set number or "L" that dictates how much bulk it is. Every 10 "L" you carry adds 1 bulk; if you only carry 9 "L" that's still 0 Bulk, and 19 "L" would only be 1 Bulk.
• There are no spell-like or supernatural abilities anymore, they are all considered spells now.
• Rarity: Things are now divided into 4 categories of availability: Common, Uncommon, Rare, Unique. Uncommon and Rare things are not necessarily stronger, they are just less common. The difficulty of identifying something increases based on its rarity.
  • Common: These things are found everywhere, and pretty much universally used. Some examples, a longsword, the ant haul spell, or a Goblin Dog.
  • Uncommon: These things are the result of special training, or part of a particular culture or place. For example, a Dwarven Waraxe, a monk's ki strike spell, the Detect Poison Spell, or the Alghollthu Master monster.
  • Rare: These things are typically non-existant. Some examples, Orihalcum armor, the antimagic field spell, or the veiled master monster.
  • Unique: These things are one of a kind, like the monster Treerazer.
• Initiative: Initiative is no longer a set thing, instead it's now a skill check that depends on the activity you were performing in exploration mode.
• Hero Points: Hero points are now a part of the base game.

Feats

• You gain at least one feat every level now, but the type of feat depends on the level.
• There are 4 main different types of feats: Ancestry, General, Skill, and Class. There are other types of feats such as Archetype feats as well.
  • Ancestry: Ancestry feats represent things you've gotten from your ancestry.
  • Skill: Skill feats are feats that are directly related to your characters skills, and their proficiency in them, these may add additional actions you can take with that skill.
  • General: General feats include skill feats, but also other things such as armor and weapon training.
  • Class: Class feats unlock new abilities and powers for a character's class. Generally, Archetype Feats are taken instead of some Class Feats.

Races

• Race is now Ancestry.
• Ancestries now give a flat amount of HP.
• Ancestries give far less abilities at the start than they did previously, but open up different routes for ancestry feats later on.
• Ancestries also give a choice of Heritage, for another choice of base-level abilities

Skills

• Some skills have been consolidated.
• Some skills have different actions available for them based on your degree of proficiency with them.

Classes

• You now gain a flat amount of HP per level instead of an amount from a die roll.
• Have a key ability Score, this is used with your class's proficiency modifier to determine the DC of class abilities.
• Have less static class features, instead most of their class abilities are determined by class feats.
• All classes besides Monks & Fighters also choose a subclass that determines a handful of features (such as a Barbarian's Instinct or a Sorcerer's Bloodline)

Equipment

• Economy is now in terms of silver pieces (sp) instead of gold pieces (gp).
• Items vary in quality both physically and magically now, instead of just magically.
• You have a set amount of magic items you can attune with.
• Items have levels, and these levels let you know what's available for you to craft (and sometimes buy).
• Poisons are actually relevant at all levels.

Spells

• There are now only 4 spell lists: Arcane, Divine, Occult, Primal.
• Spell Schools still exist.
• Vancian casting still exists.
• Heighten is no longer a metamagic, and is a thing you can do with any spell. Heightened spells vary in effect based on how much you heighten them.
• Cantrips, like before, do not use spells slots up, and are automatically heightened to the highest level spell you can cast.
• For spontaneous casters, you know spells of a particular spell level (ex: both fireball 3rd level, and fireball 5th level are different spells for a spontaneous caster to memorize).
• There is no minimum casting stat for determining whether you can cast a spell, similarly you don't gain bonus spell slots for a higher casting stat.

GM Stuff

• Monsters: The Gamemastery Guide (coming January 2020) is expected to have rules on monster creation, supposedly it's a departure from the monster creation methods of before. The new action economy allows for abilities to be created for monsters with different action costs based on their effects. For instance, a Hydra can spend 1 action to make a single attack, or it can spend 2 actions to attack with all of its heads. The stat blocks themselves are far more streamlined than before, and are just plain useful.
• Traps: Traps have been thrown into the Hazards category along with Haunts, and have been broken down into two categories: Simple and Complex. They also have much more information listed in their stat blocks that before.
  • Simple: Simple Hazards are ones that go off once, and are done. They don't participate in the initiative order, and don't make their reaction again, unless reset. These range from the humble pit trap to the typical Fireball Rune and the mighty Armageddon Orb (which rains fiery death when its trigger is met).
  • Complex: Complex Hazards participate in the initiative order, and perform their actions in typically a listed automated routine. These range from the subtle Quicksand to the common Poison Dart Gallery and the malevolent Darkside Mirror (which swaps players out with evil duplicates each round as it sucks them into an alternate dimension).