I don’t think the rehashing is the problem, but the world building. When MCU started out it was just a bunch of dudes who can punch a little stronger than the average guy or wore a special suit. Nowadays the MCU is three layers deep into multiverse, quantum realms and time travel magic and has heroes that can reshape reality. It’s very hard to care about anything when literally anything can happen and nothing follows even remotely consistent rules. Even within the same series they f’ things up beyond believe. Remember when quantum realm was this spooky thing of indeterminism? Nowadays it’s full of regular people, cities and looks no different than any other alien planet.

When the MCU stays more grounded it is still watchable, e.g. No Way Home had plenty of good or even great moments, but if you try to figure out how the multiverse logic of that movie works or how to reconcile it with everything that came after, it’s a complete unfixable mess. Also doesn’t help that they can’t follow their own setups (Thor joins the Guardians… for 5mins).