Words words words
Honestly, most of those movies felt overlong and indulgent because at least a few scenes existed just to show off the cinematography or slow-burn character shots or things like that. I mean, as much as I love movies about the space program and liked most of the actors, the last time I watched The Right Stuff I had to fast-forward through several segments that felt pointless.Let's see, of the top of my head I can think of a few 3 hour+ epics I've enjoyed in the theater (some caught as revivals) ... Schindler's List , The Right Stuff, The Godfather: Part II, Lawrence of Arabia, Gone with the Wind ...
Now, is is ambitious of them to think End Game belongs in such august company? Probably.
That's not to say they weren't good movies, or my tastes haven't changed in possibly bad ways (I get so bored with stories playing it coy when we have a really good idea what we'll see next), just that when I think of really long movies, most of them haven't been worth it.
But as some of these films creep toward 2.5 hours, and now to 3 hours, I find that what I'm missing is half a chance to breathe between action, comedy, or character shots. Like, in Infinity War (2 hours and 40 minutes long), I kept wanting them to take a few more minutes to, for example, show how more ordinary people were reacting to Thanos' invasion, or shots of the Wakandan airships doing something, or Shuri spending more time talking with Banner, or whatever. They've got more than enough characters and events to deal with. If they use this time to help tell the story in a less frenetic way, I'm fine with that.
If, however, I'm in the theater trying not to yawn as Chris Evans and Scarlett Johanssen make sadface at each other for ten minutes, then I'll definitely be unhappy with this decision.