I see three main factors:
1) General decline in movie-going as a leisure activity. And frankly, I only see that accelerating. All the cinemas near me have closed down.
2) Poor, formulaic writing. People will not keep turning out for the same story with slightly different characters.
3) The MCU has lost touch with the ground. Iron Man was set in a world that could have been our own. But with the blip, space stations, etc, the MCU has become nothing like the world we live in.
I dunno if 1 will accelerate - it might - but if it does, it'll be in part because as revealed by the pandemic, MCU/DCCU-style superhero movies are some of movies
least worth seeing at the cinema. They've increasingly rarely made actual good use of the big screen, and their wild overreliance on ultra-heavy CGI contributes to this - further, some of them actually got improved for home release, because Marvel touched up the FX or corrected issues!
The cinemas near me remain open (in North London), but their prices are very depressed compared to what they used to be, in order to get bums in seats (apart from the ones even further out in true suburbia, interestingly).
2 is the case for sure and Kevin Feige needs to eat the bulk of the blame for this. It is he who has been making these decisions on who writes and broadly what is written and authorizing these movies. I think sadly The Marvels, which by most accounts is not as bad as recent MCU movies, is suffering from something videogame sequels often do - it's not the crap sequel that necessarily suffers from a huge sales drop, it's the sequel AFTER the really crap one (there's also sexism in the mix, but it's hard to say how big a role that plays here). Notably all MCU movies have been delayed out of 2024 except Deadpool 3, which is only technically MCU. Captain America 4 is undergoing yet more reshoots (not for the first time), which will no doubt make it even more disjointed and less compelling (no doubt to lessen/remove a certain character's presence - a character who should never have been there in the first place - not every character needs to make the leap from comics to screen!).
Further to blame Feige and Disney/Marvel management, it seems like a lot of these movies should just never have actually be made, and only got made because of the perceived need for an overarching plan consisting of various interlinked movies. It's got a lot of bad to mid movies kicked out the door or even decent ones rushed out with terrible VFX/CGI because the order needed to be correct.
3 was somewhat inevitable if the Marvel universe went on, I think. You can't get away with completely forgetting to have consequences with movies in the way you can with comics. Movie audiences just aren't the same people. If they see New York trashed in one movie they do expect there to be an impact. The blip was a corner they wrote themselves into, because the only way out of letting Thanos get the snap off was a reset of some kind, and going for the obvious time-travel reset to stop him before it happened might have seemed to cheap to audience - but I don't think it would have. It's made the need for coordination of movies and so on drastically worse, so really is a huge self-inflicted burden by Marvel/Disney. You're certainly not wrong though, and the problem is Feige and the rest seem to think this is fine, and that "cosmic nonsense" is the right way to go. This is crazy because just like Secret Invasion sucked in the comics and should never have made it to screen (that it did in a version that was somehow much worse is amazing), the cosmic stuff in Marvel is generally the least-compelling and most nerdy stuff. You need someone with a touch like Gunn to get away with it, and they haven't got anyone like that.
And whilst it was somewhat inevitable, it could also have been dragged back groundwards or taken in directions that gave enough space to distract from the previous shenanigans. And Disney/Marvel just don't seem interested in that.
The grounded-ness matters too because it was, in comics and in earlier superhero stuff, the major distinguishing factor between DC and Marvel. DC had these epic, godlike, iconic characters, sure, but they often felt slightly mythic rather than relatable, despite some strong attempts. Whereas Marvel didn't have quite as iconic characters (barring Spider-Man, the X-Men, and arguably the Hulk), but it did have a universe that seemed more like our own, more relevant. They've lost that, and much as people might like, in the short term, movies full of whimsy and weirdness, and indeed, considered on their own, such movies may be strong, it detracts from one of the major selling-points. I don't think Feige and Disney/Marvel management really "got" this until recently.