In my opinion, its less of a genre fatigue and more of a "fatigue of mediocrity"

No one would be complaining if what disney was pumping out has been high quality, but it isnt.

Realistically, it comes down the piss poor management on behalf of Kathleen Kennedy for Star Wars, and Kevin Feige for Marvel. Really out of character for someone like Feige who shepherded 23 films averaging a billion dollars each in a row..

However my theory is Bob Chapek really fucked him over by mandating content for Disney+ in the wake of Endgame becoming the highest grossing feature ever.