I think Robert Meyer Burnett had a more nuanced argument lately about Star Wars. He suggests that all franchises have a maximum cultural expiration date and that Star Wars has reached it. I think there's a case to be made for that. You might also say the same thing about Star Trek, frankly.
The sequel trilogy reflects a straining to try to continue Star Wars in a meaningful way. The pendulum swung in two different directions--direct homage with The Force Awakens and cynical subverting of expectations (and political soapboxing) in The Last Jedi. Either way, the result simply doesn't resonate even close to what the original Star Wars films did from 77-83. They just don't. It reaches the point of diminishing returns. And everything I hear about Episode 9 screams "try hard". A cavalcade of force ghosts and openings of mystery boxes, all sound and fury signifying nothing.
There's only so many times you can watch a lightsaber battle or some revelation that somebody is a father/brother/sister/
boyfriend before it gets stale, especially since Star Wars itself merely took a bunch of tropes from decades past, dusted them off, and mashed them together and presented them to a new generation that hadn't been exposed to them yet.
And the same thing will happen to the MCU. You eventually wear out a genre after a while. We've had, what, 20 years or so of the modern comic book movie craze. I don't know where the genre will peak but it's probably a safe bet that Endgame's grosses will never be matched in the foreseeable future.
Anything done in excess will wear out its welcome, period, even when done well. Even Bob Iger hinted at SW franchise fatigue, did he not?
Note that the first Star Wars movies were special because you had to wait three years for each one and sci-fi movies (and TV) during that period were much more uncommon than today. Today there is sooo much competition for entertainment that even tent-pole movies just don't impress nearly as much as they used to, hence we had a string of under-performers or outright bombs this summer.
The seeds of failure for the sequel trilogy really started with JJ insisting that the old actors be utilized as much as possible. Passing the torch is really hard to do well. The more you give the old guard to do, the more you prevent the new characters from asserting themselves. For instance, Harrison Ford really carried most of The Force Awakens. But the logistical need to get the old guard off the stage leads to some contrived situations like unceremoniously offing Ford or Luke dissolving from exhaustion. It just doesn't flow naturally. When Obi-Wan died in A New Hope, we had just met him. He was not an old actor that the fans remembered from prior Star Wars movies decades earlier, so that sort of thing worked far better. It's just really hard to establish a steady continuation, one generation to the next, which is something CBS is going to have to tackle next with Picard.