You can trace the decline of The Simpsons to Phil Hartman's death. It's the start of the show turning from something clever that had jokes/situations, where one level amused kids, but also another level that only adults would pickup, to a show just aimed at visual / verbal humour at young kids through young teen level. It's why the show's kept going on, but 'new' (which can now be over 20 years old) Simpsons episodes (or quotes / scenes from them) are not the cultural touchstones of the old one's. There's nothing in the last 20 years where you can just say a word / phrase, like Monorail or Dental Plan and a large part of a generation would instantly know what you're talking about.
