You're right. It's only base dice, no modifiers.
This probably sounds terrible, however, there is a good reason:
It's more balanced than it seems.
It's hard to tell sometimes; believe me I know. But, until you get more hands-on experience with the game, you have to give the game you're playing a "grace period" where you trust that the designers made the correct decisions, even if it seems like they didn't at first. Every campaign you play or DM is going to be different. If you change players or DM, then the campaign will be completely different. Some of those games, you will have players that built weak casters, and experienced grognards that built martial types. Although it is a roleplaying game, the "player skill" of the game comes in with decision-making during game time, and character building during creation.
That said, if you GM a game and you consistently encounter the same balance issues across encounters, then yeah, you should feel confident that houseruling whatever the issue is will improve your experience.
Your Wizard's spell slots are pretty big.
Bear with me a moment and imagine that the Wizard class is actually Batman. The biggest strength of the Wizard is that they have a tool ready for every situation -- at least, if they're a good Wizard. That's the player-skill-decision-making part. The spells your Wizard picked aren't bad, they just make him very good at things that aren't combat. Your Wizard has a ton of tools for dealing with non-combat situations. Imagine if Batman didn't bring his Batarangs when he went out on a mission. He's still the best detective on the planet, but now he isn't as effective as he would be in a fight. Spells are really, really good at solving specific problems.
That said, given the nature of the encounters you described, you might try throwing your Wizard a bone if you haven't already. D&D is more than just combat, and if you aren't giving your non-combat specialist Wizard any non-combat to specialize in, he'll feel like a useless player. If your player is concerned about his build, then you can allow him to switch out a cantrip, or give him a couple of good AOE scrolls in the next loot pile to scribe.
In addition, cantrips scale at certain levels. It's not immediately, but you can be sure that the cantrip will be a good standby at later levels.
Tasha's Cauldron of Everything fixed the problem with magic stone, sort of.
You have made all the relevant observations about magic stone. It is good until it falls behind other cantrips at 5th level. So if you are planning long term, even though it is good for a little while, it becomes a waste of space later, so isn't even worth taking when it is usable. Tasha's Cauldron of Everything introduced some optional class features to the Druid and Warlock that make it less of a trap choice in the early game.
At 4th level, Warlocks get the Eldritch Versatiliy feature:
Whenever you reach a level in this class that grants the Ability Score Improvement feature, you can do one of the following, representing a change of focus in your occult studies:
- Replace one cantrip you learned from this class’s Pact Magic feature with another cantrip from the warlock spell list.
And druids get Cantrip Versatility:
Whenever you reach a level in this class that grants the Ability Score Improvement feature, you can replace one cantrip you learned from this class’s Spellcasting feature with another cantrip from the druid spell list.
Artificers have this feature built into the class already, so no optional features required. Your intuition is correct - it can see some use in the early game, but it gets bad later. If the DM is cool with the optional rule from Tasha's, just use it while it's useful, then swap it out later.
Niche uses: not worth the cantrip slot
There are still some situations where you might find yourself saying "magic stone would be useful right about now", but they are going to be pretty few and far between.
- Your martial comrades have been relieved of their weapons.
In this case, if for some reason one of your non-caster party members needs a weapon of some sort, magic stone is better than punching in melee and better than nothing at range. So you might find yourself thinking "magic stone would be nice right now", but you will quickly be reminded how bad it is when you use the rest of your cantrips far more often.
- You want to look like you're throwing pebbles while actually hurting someone.
Remember when you were a kid, and you had the brilliant idea of putting a rock inside a snow ball? That's magic stone. You can look like you're throwing mostly harmless pebbles while actually packing something of a wallop, relatively speaking. Again, not worth the cantrip slot, unless you're into this kind of mischief.
Best Answer
Baseline
Each will include damage to one target, two targets, or all targets in an appreciable area (15-foot cone) as x1/x1.5/x2 if applicable. We'll also assume that persistent damage lasts for 2.5 rounds on average, according to comment from asker. Two entries will exist for each spell not targeting Reflex, the first assuming a Moderate save (+12) and the second assuming a Low save (+9).
Cantrips Targeting Saves
Cantrips Targeting AC
As above, but we'll assume a spell attack bonus of +11 against a Moderate AC (21) or Low AC (19). We'll assume splash damage can hit multiple targets if present as above, though that's really just for acid splash's sake.
Results
Many spells targeting Moderate saves or Moderate AC do more damage than electric arc targeting a High Reflex save when facing a single creature. Only daze, gale blast, and haunting hymn require a single creature to have a Low save or Low AC to surpass.
Several spells still don't surpass electric arc in damage when faced with two opponents in 30 feet, even with a large disparity in DC from High to Low. These include daze and several single-target attack spells (acid splash, divine lance, phase bolt, and ray of frost).
Outside of those five, all remaining spells surpass electric arc in damage against two or more creatures when targeting a substantially lower DC from High to Moderate or Low.