The MCU is in such a sad state right now that there's an ever-growing amount of people now claiming the MCU was never good to begin with.
I am not one of those people, mind you, but I understand the sentiment. I was a huge fan of the MCU back in the day, and I still enjoy the majority of those previous movies today.
Hell, there's even a lot in Avengers: Endgame that I'll still defend to this day, too. Endgame is a much better movie when you watch it as truly the finale to the MCU. The more this cinematic universe carries on, the more Endgame gets totally ruined.
Never good is definitely too harsh, but I think what people might mean when they say that is that they've reevaluated it and found it lacking, not as good as they thought it was at the time.
As jaded as I've become to capeshit in general and Marvel in particular (though I also have no desire to watch James Gunn's DCU by James Gunn), there were definitely bright spots in that first decade. Nothing that I would call high art or absolute cinema, but there were some fun movies in that bunch, especially when they were still a bit more free to experiment before everything got stupidly formulaic. Some I would even consider watching again sometime.
But as a whole, as this big sprawling multi-film saga? Ehh. Thinking back, most of it is just kind of okay at best. The movie starts, your brain shuts off, you watch the bright lights for a couple hours, and then you leave feeling just generally okay with it. We were strung along with promises that things were going to build up to something bigger, and those Avengers tentpoles were certainly spectacles, but still pretty empty overall. It's the cinematic equivalent of taking all your action figures and smashing them together. The overarching plot stringing all these stories together was also pretty barebones, just a general teaser of something big on the horizon, but it still got us to go see middling movies we might not have otherwise bothered with, solely to keep up with the story, even if we didn't actually need to in retrospect. Fool me once, shame on you; fool me a dozen times...
And then Infinity War and Endgame happen, and it's like...okay, now what? How can you top a bad guy who literally just deleted half of the entire universe? Realistically, you shouldn't, because at that point the stakes have gotten so ridiculous that we can't even wrap our minds around them (and apparently neither could the writers, considering they just don't even bother exploring the ramifications of half of all life disappearing and then reappearing). But instead, they plowed straight on ahead with a whole multiversal saga, creating a conflict that's even
harder to comprehend the stakes of, and not even really giving any clues of what it's leading up to (in the movies, anyway; the studio itself just went ahead and announced all the movies several years out). And in order to get anyone interested at this point, they're jangling every single memberberry in front of the audience, reminding them of the characters they actually liked before they got replaced with diversity hires. And let's not forget nerds' greatest desire, for some reason: the X-Men! In the MCU! Wow! I'm going to fucking kill myself!
I still haven't watched anything Marvel since Endgame, and I have no desire to change that, nor much desire to revisit most of them. Or hell, any of them. If I'm gonna rewatch a movie, it's gonna be something I really enjoy, and middling okay movies aren't really worth my time.