The Zombie Simpsons explanation paints it (fairly convincingly) as a combo of deaths, turnover in the writing room, and the influence of The Simpsons and other “subversive” shows affecting the mainstream so much that it was no longer distinctive, with the result that the show became a heightened, silly, absurd, but basically straight, and (more, at least) earnest, version of the family sitcom it had started off lampooning.

https://deadhomersociety.wordpress.com/zombiesimpsons/

Under this explanation, the early show is basically a totally different thing from what it became by somewhere around season 10. Even if it didn’t “get worse” (I think it definitely did also do that, but it’s not necessary for this explanation to work) it became something so different that it’s not surprising that a lot of people who liked the early show, don’t like what it has been since the change.