First off, all of edgerunner's answers are great. But I wanted to add some Dungeon World specifics:
Check p.19 and you'll see that 6- isn't "failure" - it's "trouble". The GM will say what happens and the player will mark XP. You are attaching non-DW simulationist ideas to DW mechanics by your supposition that 6- means "failure."
These principles can apply in all sorts of games, and have been used by GMs for years. If the PCs have to climb a fence, they're just going to keep trying until they succeed, right? So even in traditional games, many GMs will read "failed" rolls as a lack of some quality - not fast enough, not quietly enough, not without hurting themselves, etc., instead of just keeping them on the wrong side of the fence.
This is because failure is boring and stops moving the story forward. So you are correct, there is no plain-old failure in DW. It's not in the GM's agenda to make the PCs fail. There is no move for failure.
So the problem isn't that edgerunner's ideas are non-optimal, it's that your concept of what 6- means is wrong and that static failure doesn't exist in Dungeon World.
Expanding on 6-
From the text:
Generally when the players are just looking at you to find out what happens you make a soft move, otherwise you make a hard move.
Somewhere in Apocalypse World itself it says about hard moves:
make as hard and direct a move as you like
Early PbtA games like DW assumed you understood Apocalypse World. And this phrase is often tacitly implied in PbtA games even today.
6- means trouble as I said. The GM is free, on 6-, to make a move as hard as they like. That doesn't mean as hard as you can think of.
AW says:
It’s not the meaner the better, although mean is often good. Best is: make it irrevocable.
So while a 7-9 should substantially give the character what they wanted (they accomplish their intent even if their action created complication), on 6- you are free to deny the intent (the action still has to have consequences beyond "no" though) and in addition make a move as hard and direct and irrevocable as you like.
Climbing a mountain a soft move is "The boulders above you on the rock face begin to wobble as the grappling hook you've tossed up there sets itself. What do you do?"
A harder move is "The boulders have tumbled off the edge of the ledge and after hanging nearly motionless for a tiny instant above you, are now plummeting towards you, gaining speed every moment. What do you do?"
A really hard move is "The boulders are yanked free by your grappling hook and come smashing into you, tearing you from your narrow perch and scattering the contents of your pack into the yawning emptiness beneath. What do you do?"
Let's have another look at the Know-It-All move (emphasis mine).
Know-It-All
When another player’s character comes to you for advice and you tell them what you think is best, they get +1 forward when following your advice and you mark experience if they do.
Now let's look at your proposed scenario: the Fighter is under attack and you've asked him the all-important question "What do you do?". In this scenario there are plenty of barriers to the Fighter getting his +1 forward and the Wizard gaining his experience. They could be overcome if the fiction allows it but would take some doing.
- If the Fighter (the character, not the player) tries to ask the Wizard's advice, I'd say that's a golden opportunity to have the enemy take advantage of his distraction and follow through on whatever injury they were about to inflict. Even if the Fighter manages to shout out a question, by the time he gets an answer it'll probably be too late.
- Could you even consider a shouted question in a desperate situation to be "coming to the Wizard for advice"? I certainly wouldn't say that yelling "What do I do!?" would count and thus the triggering condition for Know-it-all isn't met.
- Even if our Fighter somehow overcomes the first two obstacles, there's still the problem that the Wizard can't just tell him to "Defy Danger!". Not only does such a phrase leave the fourth wall in tatters but it's not even advice that the Fighter (again the character, not the player) can act upon, so no +1 forward and no experience marked.
All in all, what you should remember is that Dungeon World is always Fiction-First. Your players don't tell you what moves they're using, they tell you what their characters are doing. If their fictional actions trigger a move then you as the GM let them know that they're about to trigger a move and ask if that's what they want to do.
Best Answer
Mind Your Follow-Through
Note that Defy Danger starts out with:
Getting rushed by screaming kobold fanatics who've set themselves on lightning* is pretty calamitous, and if you hit on a 10+, that's great! But what it means is that the threat doesn't come to bear right now, not that it vanishes forever - "defy" doesn't mean "eliminate".
Yes, you are encouraged to start setting up the scene with softer moves, but all of these moves are things that are actually happening in the fiction. The players aren't trying to, like, counter and eliminate your moves or anything, they're also taking actions in the fiction and accomplishing their own things. They can manage to be both momentarily safe and still very much in danger.
So, you hit on a 10+. The screaming kobold fanatics haven't gone away. It's just that when the first one rushes into the innocuous-looking jar/carefully positioned Leyden mine and they both go up in a cloud of electroclasm and smoke, you're not right there with them.
Awful lot of jars in here, aren't there?
* no one ever said the path of the blue dragon fanatic would be easy
Mind Your Setup
Tell them the requirements or consequences and then ask is often underestimated, but it's an excellent way to extend the results of moves without having to write entirely new ones.
So if, for example, Stringfellow surveys the upcoming holosparkst, decides it's time for the better part of valor, and says he's diving out the nearest doorway, that does sound like Defy Danger too, doesn't it? But you can always say:
Or:
Or even:
And then end with:
And if it's not, then Stringfellow isn't going to dive out the door in the first place. His turn in the spotlight will be spent doing something else. And if it is, then even a 10+ on a Defy Danger will still leave Stringfellow in a bad position. It doesn't violate the spirit of the move to do this - accepting those known bad outcomes is just the cost of making the move in the first place.
Mind Your Prep
And, of course, if you knew all along that there'd be kobolds in these ruins and they laid traps like the dickens, you can sit down and craft custom moves to deal with this and take Defy Danger off the table as an initial reaction. Something like:
But you don't need to haul something like that out all the time, just for when you want it to be a sufficiently big deal that a regular Defy Danger doesn't seem like it should be able to resolve things satisfyingly on its own.