The reason the multiverse concept took off in the comics is because their storylines got so bloated with so many different writers working on titles featuring the same characters that they ended up with tons of "canonical" plots without any semblance of global continuity.
As this happened, readers lost interest because they were unable to follow everything clearly. Events that involved Superman or Batman in one title had nothing do with whatever was going on with them in some other title. Eventually, it just gets really frustrating to be a reader in that environment, because you have to either:
- Keep up with every comic ad infinitum; or
- Just accept that the storyline from the series you're reading has absolutely nothing to do with the stories someone else is reading about the same characters.
Neither solution is great, so DC/Marvel did stuff like "Crisis on Infinite Earths" in order to (diagetically) cut back their titles to just the most popular ones and merge them all into a single timeline.
But that isn't really satisfying for readers. For one thing, it means that some fans' favorite stories are no longer considered canon and some versions of characters they really liked end up disappearing entirely.
And no matter what, the idea of infinite universes inherently means that no story's stakes are meaningful. Even if Thanos or Darkseid destroys the entire universe, there will always be another one... and wouldn't you know it? The other one is nearly identical to this one, with all the same people and superheroes in it doing virtually all the same things.
Multiverse retcons are also not a sufficient solution to the core problem.
The publishers never appoint an EIC who maintains a unified continuity across all future books, so the problem just returns a year or two later and they have to do it again.
The MCU had the opportunity to avoid all this junk by keeping everything in one single continuity. Even though were always lots of movies and shows in the queue, it was actually pretty easy to do compared to the comics, and for the first 10 years, they handled it pretty well (thanks to Kevin Feige acting as that single voice of continuity across all the films).
Even the Netflix series were basically in the same continuity, and although they abandoned Inhumans and never did anything with Ghost Rider, Agents of SHIELD did okay for a while as well.
That all changed in Phase 4, where they decided to go all-in on "multiverse" stories, even though they really didn't actually have a practical meta-need to do so.
The "benefits" of the multiverse are almost exclusively more opportunities nostalgia-bait (see also: The Flash bringing in Michael Keaton, CGI Christopher Reeve, George Reeves, Fleischer Superman, Nic Cage fighting a giant spider, etc.); but that gets annoying really quickly and the price you have to pay in continuity and meaning aren't worth paying at all.
You're going to see a lot of people writing more and more articles about "superhero fatigue", but that ain't it at all. The fatigue is coming from the fact that the way Marvel is choosing to tell stories now took away our reason to care.