There seems to have been a big turnover of writers in 98-99. As in very few people who were there in the beginning were left by that point. You're not considering that the original charm of the show was lost with the original creative force.

Family guy debuted in 1999. It's hard to say Simpsons tried to copy family guy's style. Family Guy is really known for its cutaways (usually to some non sequitur) & somewhat crude humor. A lot of the jokes at this time hinged on Stewie not being understood by anyone but Brian, Brian himself being a dog. There were also a lot of references to musical theater. The Simpsons was different from this.

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.