QoL

[Hakuro89]

IIRC repairing other people's gears existed in 1.x but it was removed. As much as I want that functionality back, I can also see why. It's too prone for abuse.
For starters, it absolutely should not take anything from the crafter repairing, otherwise people will just say no, even if they're compensated. At the very least, it means they'll have to go get more later which is a hassle, but they might only have enough for themselves. You should manage your own dark matter when repairing gear, even by proxy like that.
Then there's people who don't know how gear repair works, and will start harassing people for not having certain crafters so they can't repair their gear. "But you have a level 70 CUL!" yeah, for all the good that'll do your broken boots...
And then there's the simple fact we're dealing with the human element. They can say 'no' for a plethora of reasons even if all it takes from them is clicking a button - and let's not get into how forcing a dialog on someone is already problematic. You can't even trade in instances.

And then there's the other issue where if you do anything that isn't just slapping a mender NPC in all instances, you're effectively forcing crafters into the battle meta...

The simple, go-to solution regarding this would really be to slap a mender in instances, but that would make gear durability even more redundant, they might as well just remove it

You... don't really have an argument here. You just described a system that already exists in the game.

Materia melding via other players already exists. Which is also job/level dependent, meaning the melder has to have the correct jobs leveled up to at least the level of the equipment (not so for repairs, which are up to 10 levels below the item's level).
On top of that, melding also requires a catalyst, carbonized matter, which is taken from the person requesting the meld. Nothing is taken from the melder.

My point is that a system that tweaks gear at the request of another is already in place, and very little additional work would be needed to take this a step further for repairing gear, mirroring the existing Request Meld system.