I still enjoy the new episodes.
I think that people who like Family Guy (as, for some reason, it is always compared to that show) just haven't ever understood the multi-layered nature of humour on The Simpsons. The more you know, the funnier it's always been; a lot of people might see only part of the hilarity because it's made to be funny for multiple audiences at a time.
Yes, the show got more outrageous with age, but I actually enjoy that change. You have to keep things fresh somehow, and I believe that the show has grown with the times. It hasn't "run its course", because it's still coming up with new material, despite shows like South Park directly accusing it of having "done everything". There are new things to comment on every month, and the show manages to do that without feeling old. Like the funnies in the newspaper, there is something very integral about this show as a home experience, not just in America but in Australia as well.
As far as I'm concerned, there is absolutely no reason not to keep it going. Despite the rampant complaining from people who quite possibly avoid watching it anyway, the show remains very popular and very lucrative business-wise. If Matt Groening or any other critical voice in the making of the show wants to quit, it won't be because he isn't getting the profits he wants. It'll be because they are tired and have worked on the same thing for more than 20 years.
While I respect the OP's opinion as someone who has seen the entire series so far, I believe that he may just be tired of it because he's watched the whole thing rather than objectively seeing a decline in quality. The Simpsons is, in my view, too big to do that and not get jaded. That's why I think it's more suited as a TV show you end up watching because it's on, rather than something you buy the DVD collections for and watch in a binge like Heroes or Lost. It's possible to do it with those shows because they have a plot progression over seasons and ultimately the whole series, something The Simpsons just doesn't have. The whole of Springfield is in a whacky time-warp junction where everyone celebrates Christmas 20 times and doesn't get any older.
So it's a no from me. It shouldn't be cancelled, people should just stop watching if they are bored. There are plenty of other people who will probably take their place.