Also, he has to know that a large part of the city governance comes straight from Albany. We can't change most taxes without the blessings of the state legislature, and they control a lot of what goes on down here, especially in terms of fiscal stuff.
He'd also be prevented from making all those ridiculous speeches which have greatly enriched him.
I really don't see him running for office again -- I think he just wants to be a big old pain in the ass to whomever is in office and getting any attention that he wants for himself. I think he wants to be seen as a wise, elder (not too elder, as that's a turn-off for the babes) statesman.