Bill Murray should have quit? Are you serious? The man keeps getting better. Look at what he has done since he peaked according to you:
Scrooged
Quick Change
What About Bob
Groundhog Day (Should have been nominated for an Oscar here)
Kingpin (one of his best comic characters ever)
The Man Who knew too little (Dumb movie, yet Murray was hillarious in it)
Wildthings (T&A / sex thriller, but again Murray steals it)
Rushmore (again, he deserved an oscar for his portrayal of Blume)