In May 1997 The Simpsons aired an episode entailed "The Simpsons Spin-Off Showcase", as a satirical satire of how TV producers allow their own shows to continue running long after they have past their prime, resulting in cheesy spin offs and other lame attempts to keep a show alive. Unfortunately the creators of The Simpsons ignored their own warning, oh the irony!
Explaining of the vary reasons of the problems with The Simpsons nowadays almost seems redundant, ranging from poor writing to over reliance on visual jokes.
Everyone seems to have their own opinion on when The Simpsons entered the entered the realm of unwatchabilty, beginning with the infamous episode "The Principal and the Pauper" (which personally I don't think is as bad as it's reputation makes it out to be, but that's for debate), to some time in the early 2000's. Reagrdless, I think most of can agree the show have ended years ago.