In case anybody is wondering:

The Simpsons started to go downhill when they started to trying to change things permanently. You know, like Apu getting married in one episode and still being married in the next. Maude Flanders dying and staying dead. (Bleeding Gums Murphy and Marvin Monroe don't count because.. well you know whu). Lisa becoming a vegetarian or Buddhist and staying that way. Stuff like that.

The Simpsons being a cartoon, the characters don't have to age. Nobody has to die. They don't have to add new cute children when the existing ones grow up. In short, cartoons are inherently immune to most forms of shark jumping. The writers/producers decided to voluntarily afflict change - the curse of live-action TV - onto one of the best animated series ever. (Yes, voice actors can quit the show or die or something, and both have happened to the Simpsons, but you can't blame the Simpsons problems solely on that.)

This has been my oft-stated opinion for years, and I'm sticking to it. Sure, the show was inevitably getting old. Sure, its better than having no Simpsons at all. But almost anybody can tell you that the 4th or 5th season was better than the 8th or 9th, and you've gotta draw the line somewhere.