I haven't listened to WFAN for a few years (moved away from NYC area), but the Imus departure has got to hurt the station big time. It seemed pretty clear that Imus attracted the lion's share of advertising dollars...in fact, his 10AM signoff used to be an extended put-down of the remaining 20 hours of WFAN programming, in terms of both profitability and content.
I know nothing about Mancow, but doubt that a new personality could be successful without NYC roots. Imus went way back as a NYC DJ (WNBC? I remember weird chemically-induced rantings in the early 1980s. He went to Cleveland for a while, but was regarded as a NY figure).