We get a fair amount of traffic here from random message boards where The Simpsons comes up in conversation and someone posts a link to us. This is often in the midst of one of the usual spats about when the show fell apart or whether or not Family Guy is better. About a month ago I was reading through one of these threads (though I neglected to save the link, whoops) and someone posted an excellent quote from Bill Watterson, the guy behind “Calvin and Hobbes”.
Thanks to Google, I was able to find the quote (bold in the original):
Readers became friends with your characters, so understandably, they grieved — and are still grieving — when the strip ended. What would you like to tell them?
This isn’t as hard to understand as people try to make it. By the end of 10 years, I’d said pretty much everything I had come there to say.
It’s always better to leave the party early. If I had rolled along with the strip’s popularity and repeated myself for another five, 10 or 20 years, the people now "grieving" for "Calvin and Hobbes" would be wishing me dead and cursing newspapers for running tedious, ancient strips like mine instead of acquiring fresher, livelier talent. And I’d be agreeing with them.
I think some of the reason "Calvin and Hobbes" still finds an audience today is because I chose not to run the wheels off it.
I’ve never regretted stopping when I did.
If The Simpsons had finished up after nine or ten seasons it’s not hard to picture alternate reality Matt Groening saying the exact same thing.
charliesweatpants.com
(The crotch really does wear out too fast.)
Dead Homer Society 
Sadly, the Simpsons’ writers think they’re still as funny as ever and running the show into the ground has become part of the joke.
Calvin & Hobbes was a single person’s effort. The Simpsons has an entire team creating each episode, but the quote still holds true. The ones that felt like their time was done…left. Aside from Groening himself, there aren’t many people from the original crew left on the show, and even Al Jean & Mike Reiss were gone for several years before returning.
And Bill Watterson also famously refused to authorise “Calvin and Hobbes” merchandise. Imagine an alternate reality in which “The Simpsons” not only ended after ten years, but there’s no deluge of crappy “Simpsons” merchandise.