Eh, it’s pretty hard to say. Being harder to fit into someone’s schedule probably did hurt endgame on the margin — how could it not? Titanic, rotk, etc etc.
Now you’re right that really long movies are often some of the best performers of all time despite that, so it’s not a big issue by any means. But a lot of the time I think that’s a common confounder — the movie is very “epic” (either some historical epic or franchise finale) which both generates a long ass runtime and a very high desire to see it, which more than outweighs any more direct causal harms from being mega long.
If Batman feels like a movie that needed to be 3 hours, I agree it’ll do gangbusters according to historical precedent. If it feels like a movie that’s 3 hours but didn’t need to be, that’s where you can have issues on net.