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.
Every movie is literally the same at this point, the ''bad guy beats good guy, then good guys gets power up and in the end beats bad guy shit''. Besides new Spider Man being good, honestly, they should have just stopped at Endgame. It was fun for some time, but after Endgame, they don't even have more ideas.
Everyone is complaining about special effects and trash scripts because they can't come up with anything else. I won't even comment on DC. I just don't know why it can't be in levels of Tobey Maguire Spider Man and The Dark Knight trilogy?
i'd say DC has some pretty big misses but can also churn out decent movies every once in a while. The new batman movie is very good imo, far better than anything marvel has produced in the last 15 years - just on the account of it having SOME creativity compared to marvel's lack thereof