The article focuses on the production shortcomings and lacking quality of subsequent phases as being part of the decline, but I think there's another explanation elsewhere.
Prior to the large extended MCU project super hero movies were kinda trash. They were the kind of trash that only really attracted bad movie fans or people who grew up reading the corresponding comics, but then Iron man came out. Not only was it not trash, it was different from the other non super hero offerings at the time as well. It was fresh and fun, and it attracted audiences who didn't grow up reading the comics.
So they made another.
And another.
And another.
And another.
They just kept making the same thing with different characters. After so many movies of just watching different colored gods shit on the bad guys with the full knowledge that sequels were already in the works so nothing permanent would happen to our handsome heroes the movies became repetitive. They weren't fresh anymore, and the fun relied more on quips and one-liners than the raw story and action.
People got fatigued.
Then Endgame came along and promised a nice off ramp--see the culmination of this saga you've been intermittently watching the last decade then you can be done.
So people watched the end, then decided they didn't want to get involved in another decade of the same story, and never watched an MCU film or series again.
Personally, I never would have seen the first phase if it weren't for watching some on a plane and Moviepass overlapping with the phase allowing me to go see some of them for basically nothing. I stopped watching after Endgame, as did most of my friends, not because everything after was bad (we had no clue, we stopped watching), but because we wanted different stories.