I like that the writer of that article contrasted the MCU with Seinfeld, because that was something that had come to my mind before. Jerry Seinfeld declined NBC's offer of $5 million an episode for a tenth season (over three times higher than any TV offer before!) because he felt it was better to end strong than to peter out. The result is that Seinfeld remains a timeless sitcom that still brings consistent laughs no matter what episode you watch.
As I mentioned earlier, a good comparison to the MCU's current state is The Simpsons, currently on its 33rd season with season 34 already greenlit. Most everyone agrees that the show's quality declined somewhere around season 8; some place it earlier, some later, but generally it's in that region that it starts to get noticeable. Rather than call it quits and go out strong, they kept going on and on, season after season. The talent that drove its wild success left for greener pastures, and the show slid into mediocrity. Nowadays, if you catch an episode at random, you're overwhelmingly more likely to watch a Zombie Simpsons episode than a classic, to the point that many younger people who didn't grow up with it don't really get why it was all that groundbreaking in the first place.
I feel you could do the same for the MCU, where you could make arguments for multiple points where the quality began to slide (Joss Whedon's quippy Avengers becoming the template for every character moving forward; Black Panther/Captain Marvel for the beginnings of woke politics; Endgame for laying the foundations for the time travel multiverse bullshit that's plaguing the series now). Regardless of when you feel the decline began, Endgame was a good place for Disney to end it, with a fitting title and all. Consoomers would cry about not getting their Marvel fix, but Disney could have gone out on top with the biggest film series of all time. Instead, we're beginning the descent into mediocrity (well, more so than usual), with movies that are met with more of a "well that was fine, I guess" than a "best movie EVAR, I can't wait for next product!" And over time, that tarnishes the opinions of the series as a whole.
In other words, cheerlead all you want, Bob. You can't polish this turd (heh, E-turd-nals).