I think that's probably why he quit. There really wasn't anywhere else to go from there. Growing up, I remember a LOT of people enjoying Seinfeld because they thought the characters were charming, or because they related to the characters. (Ick) And that was pretty much the point of no return where their pettiness was so abhorrent that any plausible deniability that these were remotely decent people was tossed out the window.The show crossed the Rubicon of meanness in Larry David's final episode before he returned for the finale, where George's fiance dies and none of the characters really care much.
I think that's honestly why I enjoy Curb Your Enthusiasm more as a show. Where the characters are unambiguously bad and vile people, almostly entirely divorced from regular life, and the joy/catharsis comes from when Larry and his schmutzy friends get their regular comeuppance for what they've done. That, and the show has been taking regular hiatuses for even years on end, to give Larry enough time to think up good ideas worth making new seasons about, instead of just forcing mediocre content.