GeorgW said:
It'd be nice to see it go on for 100 years with different actors and writers.
But it'll probably end as soon as one of the actors retire.
Funny that you mention that. I live in Spain, where the simpsons were kind of a big deal when one main TV channel started airing its episodes between 14:00 and 15:00 hours (lunchtime, basically), where the show is still frequently leading the TV share for most of the week. However, about the eleventh season the voice actor for Homer Simpson died, and was replaced for a way worse (in my opinion) colleague. Thanks to that sad coincidence, it's really easy for me to pick the good episodes from the bad ones just by listening to Homer. What I want to say is, you probably wouldn't like an actor change, and that's probably why the show would die the moment a voice actor retires.
Anyway, what I think is killing the show is the script. It has gone from brilliant stories written by hard-hitters like Conan O'Brien (yeah, THAT Conan O'Brien) to a formulaic mess that doesn't even try to even introduce the stories (You, Homer, get my sports car to Italy! What? No reason to be honest, we just thought you might have some zany adventures in there) and has long ago sacrificed any character depth and coherence in exchange for making episodes more "funny" (the non-funny kind).
As for why did they do that? Ask Simpsons' show runner since season 9, Mike Sully
When asked in 2007 how the series' longevity is sustained, Scully joked, "Lower your quality standards. Once you've done that you can go on forever."
Also:
RJ Dalton said:
The show is dead now and all that remains is a corpse animated by the radioactive power of green papers with the faces of US presidents on them.