I'm not even sure if Al Jean even likes it anymore. IIRC, he openly said he'd consider actually ending the show once they got past the thirtieth season and then the Disney buyout happened.
Al Jean is doing the show for no reason other than sheer unfiltered spite.
When you get down to the root of the issue, Zombie Simpsons is all just one colossal ragepost in TV form and stretched out over twenty years.
The Silver Age of Mike Scully was at least enjoyable in a goofball sort of way and I'll defend Seasons 9-12 any day of the week. Even the awkward "middle years" of the early-mid 2000's that were after Season 12 but before the movie were more mediocre than bad and you still had a few guys like Swartzwelder who hung in there for a few more years.
Pretty much this. At this point, Al Jean is continuing the show purely to "own da nerds". Said nerds were on Usenet in the 90's and likely don't even watch the show anymore or even know who he is.
It's every bit as autistic as it sounds.