Michael is the anchor of the family AND the show. Without him, AD would just go flying off into the stratosphere and become Stella or something.

Plus, you could argue that Michael is the most effed up of them all. The fact that he hasn't disowned these people yet (along with his obsessive, misguided quest for father/son bonding) is proof enough of his insanity. He represents the cynical charm of AD. It's a family sitcom where the family *is* the central conflict.