I think the problem is more that they keep trying to top themselves and there's only so far that can go. It's a basic comic book problem, which makes it entirely understandable when it comes to Marvel. You start with a basic superhero. They face problems and have to dig deep to overcome them and then... they need to make the next enemy stronger. Then the hero has to get stronger. Lather, rinse, repeat until you've got Superman juggling planets and they have to reset the whole thing back to square one and do it over again.
In the very first movie, Iron Man faced a very simple enemy. It was a guy in a suit up against a guy in a suit. Then it just kept escalating until you were facing gods in the first Avengers movie, until you got to Thanos and the potential end of half of the life in the universe. Where do you go from there? Audiences don't want to go back to watching Tony Stark, or whoever you replace him with, punching a bad guy. That's one reason why, I think, in Phase 4, they started sidetracking into other things that weren't trying to top the last movie. The fact that identity politics got thrown into the mix and that alienated a huge part of their audience, I figure it was just them looking for something to do that wasn't "the universe is ending... AGAIN!" That's why the threat level kept getting smaller and "the message" kept getting bigger. Kevin Feige had no idea what else to do and he still doesn't.
Comics have never figured out the solution to the problem and no "cinematic universe" is going to either. You either need to reset everything, or, better, not grow as fast as they did. They didn't do the insanity that DC tried to do, but these things should take many, many years, over many, many movies, with only small changes along the way. Marvel didn't do it and now, I don't know that they can go back without losing their audience.
Granted, they've managed to drive away most of their audience now, so I'm not sure how much it really matters.
