Audience fatigue is a fact.
The 2000s was all about fantasy/high fantasy (Pirates of the Caribbean, Harry Potter, LOTR, Chronicles of Narnia). Then in the late 00s-10s we had the vampire craze (True Blood, Twilight, Vampire Diaries) and other supernatural stuff. Then came the zombie craze kicked off by The Walking Dead which has now fizzed out.
The Marvel stuff has been going since 2008 and it is very much true that people are starting to burn out on it. Their next phase will take the MCU to nearly two-decades of inter-connected content. The fact that it kept its legs going for so long was impressive, because few fads last this long.
Most of those phases ended because the stories they are based on ended.
As for the MCU, and superhero movies in general, the bad quality is even more of a fact. If something is good people will watch, word of mouth spreads and fomo sets in. Which to an extent is why the interconnected nature of the Marvel movies worked so well.
After End Game quality really took a nose dive, not that there weren't a few duds in those earlier phases.
Now combine this decline in quality with the increase in quantity and that's what caused this "fatigue".
Most audience don't even bother to go watch a new Marvel movie because they know it will be "Meh" at best. At which point apathy starts setting in.
Spider-Man:
No Way Home for example was massively successful, proving that people still enjoy these movies
if they are good.
There are innumerous YouTube fan channels analysing and chronicling this decline for the various IPs, for those who bothered following it over the years.
It's completely different from getting bored of the same thing as if you're eating lasagna every night.
Take a look at the comic book industry these movies are based on. They have been popular for many decades, but a decline in quality a decade or so ago is what really gutted the industry. They became a shell of their former self and readily lose out to Manga.
Now we're seeing a resurgence of independent comics precisely because people got fed up with the crap marvel etc were pumping out.
Saying this is mostly due to audience fatigue is just a sad attempt to excuse the failures of the various film studios which is the root cause. I'm sure they are thankful to all the outlets that promulgated this message for them...
However that point is now moot I suppose. It will take years of concerted effort to try and turn this around.
This isn't just the case for the MCU, but the multitude of other beloved IPs that have been gutted due to lack of quality/Bad writing (Star Wars, Star-Trek, Marvel, Dr. Who, etc.) over the years.