Amph Family Guy REALLY needs to be Axed

Does that explain the decline in quality during that time?

Apart from the movie

 

Ehhh...you could try and make that argument. But Al Jean was also co-showrunner with Mike Reiss in seasons 3-4 and has been a writer for The Simpsons since Season 1. Season 4 specifically, is often hailed by fans as the best season. Jean originally left the show to create The Critic but returned when that was over and eventually took over as the sole showrunner.

Showrunner breakdown for the first 21 seasons.

 

If you've ever wanted to read entirely too much about the decline of The Simpsons, Dead Homer Society has a lengthy analysis: http://deadhomersociety.com/zombiesimpsons/