Mainly because a lot of the problems these guys are talking about just not materilaizing within the franchise have existed since like 2012.
Too many quips? Avengers 1, Dark World and Age of Ultron were infamous for this.
Every new movie is a sequel hook? Iron Man 2 was a 3 hour trailer for the Avengers, and this series popularized post credits scenes
Bad Special Effects? Phase 1 films looked like they were cooked up in about a week because their sets were so cheap.
Bad writing/Too much bait and switch? Remember how mad fans got over Iron Man 3 with the Mandarin twist?
I think all this discourse is less about Marvel “not being good anymore” and more about people starting to unconsciously realize that mindless corporate shlock isn’t actually that gripping once you start holding it under the microscope. Like this series has been going for 16 years and hasn’t actually changed, of course you’re sick of it.
Phase one seems like it had the most effort put into it. The movies had to stand on their own because it was a new project that couldve failed, and the music was also fairly memorable. It was neat how each movie only had small hints about other movies/heroes, so you got the point they were part of the same universe but it never distracted from the movies themselves. The characters also complete their own character arcs in their phase 1 movies, too.
Everything else was just crap. Once youve seen phase 1 you’ve seen everything.