Yesterday Digital Spy put up an interview with longtime Simpsonista David Mirkin. (He worked on the show from the beginning pretty much through the start of Zombie Simpsons and then on-again, off-again in more recent seasons.) It’s an interesting read, including the welcome information that we’re a long way off from a second movie. Here’s the part that rankles:
Matt Groening said that he doesn’t see an ending in sight for the show. Do you agree?
“I pray for an end! I would like to meet another human being who doesn’t work on the show! And maybe say ‘hi’ to a girl. It’s all up to the fans – as long as they’re telling us that they’re happy, they like it and they want more, our feeling is that we will endeavour to give them new stuff.
Ten thousand Thank Yous for all your work and we love you (in whatever way you’re most comfortable with), but please understand that we are un-happy and would not like any more.
charliesweatpants.com
(The crotch really does wear out too fast.)
Dead Homer Society 
I believe that when David Mirkin became show runner (season 5), “The Simpsons” became noticeably more wacky than before, which was no bad thing at the time, but it sowed the seeds for the destruction that came later on down the line.
I have to ask: is it just a coincidence that Zombie Simpsons creeped in when the show relaxed its policy on show runners changing every two seasons? Thank about the history of show runners. James L. Brooks, Matt Groening and Sam Simon for seasons 1-2. Al Jean and Mike Reiss for seasons 3-4. David Mirkin for seasons 5-6. Bill Oakley and Josh Weinstein for seasons 7-8. Then, for whatever reason, the infamous Mike Scully took over for an unprecedented four seasons (9-12), never mind that it was one show runner instead of two or three, and then Al Jean’s been solely running the show from season 13 onwards (eight consecutive seasons so far). Not that I’m saying getting a new show runner would be enough to stop the rot…
I have no inside information or anything, but my strong suspicion is that the whole “Who is the show runner?” thing has always been overblown. It makes for neat delineations (which in turn makes for easily defined arguments), but there aren’t any really clean lines in terms of how the episodes go. The one thing I’ve noticed most is the fact that once you start getting into Seasons 7+ a lot of unfamiliar names begin popping up as writers, consultants, producers, etcetera. The people who really made the show what it was gradually left and that’s really all she wrote.
On a gut level it’s a very unsatisfying answer because there’s no one person to blame, there’s no one moment you can point to and say, “Ah ha! This is why the show sucks now.” Swartzwelder, to take a famous example, was still working on the show as late as Season 15, but was credited much less frequently. The same can be said for a lot of the most productive guys from the early seasons, some stuck around longer than others but by the late 90s their ranks were really thin.
You combine that kind of brain drain with the show laboring under an increasingly complex mythology (which they sometimes ignore and sometimes treat as holy writ) and the result is Zombie Simpsons.
No one person to blame. Zombie Simpsons is like a bastard child no one wants to take responsibility for.