I'm not really moving the goalposts. The OOC arguments basically just mean that if somebody brings a Fighter to the table, you really just have to go "Fuck it, fine" and work out how your character is going to justify, in-game, bringing the Fighter along while he's adventuring. Because while you could certainly say, "My wizard doesn't see the point in adventuring with dead weight", that's a lot likelier to end with you not being invited to the rest of the sessions than it is to convince somebody that playing a Fighter isn't a good idea. So yeah, the Fighter being there is metagame.
Which is where all of the other arguments about Fighters not being as much of a resource-sink as you make them out to be come in, because those are the IC ones that justify the conclusion OOC stuff already arrived at, and which is probably more important than whether or not your fictional Wizard is slightly less omnipotent than it could be. They're just designed to give in-game plausibility, in the same way that the fireball is an in-game explanation for why everything in a 30 foot radius just took a bunch of fire damage. It is not always true, in absolutely every situation, that the Fighter is going to be able to contribute, but in the games I play it typically works out that the number of situations a Fighter actively ruins is very, very, very small compared to the number of situations where the Fighter at least contributes a pile of hit point damage. Actually, I've not seen one we haven't been able to at least plan around (recently, I've been that non-stealthy character in a sudden stealth scenario, and I just decided to be a giant distraction and it worked out okay).
Again, none of this makes the Fighter a good class. None of it makes it powerful, a viable contributor to an optimized party, or any of that. But compared to telling a friend their character concept is unworthy of existing in the same party as mine, being less awesome personally in order to make the party overall still function has always been the superior option. And at this point, we're not talking about "Here's how you can make your character better", we're talking about entire concepts. If the party is not entirely made of full casters, we will not have encounters designed for a party full of full-casters, because I don't play with douches for DMs if I can help it.