If you view the point where that show started sucking to be around the year 2000, then the show has sucked longer than it's been good.
Necro'ing this post a bit, but that is absolutely when it happened.
In the UK, the Simpsons was only available on Satellite or Cable until the mid-2000s (I remember when Satellite TV was new and having Cable was like living in Star Trek, fuck me I'm old). When I went off to university in about 1998 the Simpsons was the best show on TV and worth getting Cable for all on its own. My parents had Cable in their house so I got to see the Simpsons and the X-Files and it was fucking amazing, but being a penniless student I had to go without, and continue to for a couple of years after I graduated, living in the city where I went to University doing various temp jobs to avoid growing up.
Anyway I moved back in with my parents in about 2004 before getting a new job and girlfriend and moving out again. But in that time, I had cable again, so I was damn well going to watch the X-Files and the Simpsons, fuck yeah! The X-Files had the T-1000 instead of Mulder and was complete garbage. I didn't get to the end of the first episode. But never mind, there's always the Simpsons! I turned it on, and within the first minute I witnessed a joke that wasn't funny. It just whiffed. Complete miss. Cringe, even. For the first decade of the Simpsons' existence, I had never seen a joke fail like that. I was genuinely shocked. Then it happened again. And again. And the entire episode was like that. I don't think there was a single joke in there as good as even the lamest joke in the 1990s. It was like witnessing a beautiful woman turn into a wizened old hag in front of your eyes.
It's the kind of thing that reminds you of your own mortality, in a way. Before you hit, say, your mid-20s, you're not really aware that the world will always change and leave you behind. In fact the Simpsons' writers grasped it so well when in Homerpalooza (one of my personal top 5 episodes) Abe says "I used to be "with it", but then they changed what "it" was, and what I was with wasn't "it", and what was "it" was weird and scary to me. And it will happen to YOU." That speech was in the context of Homer not being able to understand the music Bart and Lisa liked - which was bands like Cypress Hill, Smashing Pumpkins and Sonic Youth. Sonic "Youth" are now in their 60s. The classic Pumpkins albums are now OLDER than Homer's "dinosaur bands" (Grand Funk Railroad, Jefferson Starship, the Alan Parsons Project) were when that episode was made. There are people out there who not only consider Siamese Dream to be "dinosaur music" whose OWN tastes are considered to be "dinosaur music" by Zoomers. "Hey Ya" is 15 years old.
Fuck. Abe was right. It happened to ME.
This essay brought to you with the power of Sleep Deprivation (TM)