I get what you mean. And fanservice.....that's a trick, right? A little bit can make a movie more fun in a guilty pleasure sort of way, but too much can destabilize what you're trying to do and make the whole thing a mockery.
But I think Endgame earns a free pass. This wasn't just any movie, and as the farewell to so many characters, actors, and the entire Infinity Saga I think all the fanservice was not only acceptable, but actually kind of warranted. Even the "female heroes team up" works, I think. Yes, it's utterly unlikely and ridiculous that these specific people would all be in that one specific place at that time, but so what? The MCU (and films in general) are so male-heavy what's the harm in a three minute scene of girls kicking ass together? When I saw Endgame in theaters, every woman in the place gave a cheer at that scene (my wife included) and you know what? They deserved that. The female fans of the MCU deserved that. I'm glad they got it, even if it doesn't *really* make any sense.
Obviously YMMV and this level of fanservice isn't something I'd usually agree with, but......its Endgame. F*ck it, go for it!