PFnV, the quote is as follows:
"Yeah, you're a genius, everyone knows it, a goddamn genius, but that's why you failed as a head coach -- that's why you'll never be a head coach...some genius." According to Halberstam it was said over an open channel during a game.
What are you talking about? Parcells worked as a college coach for 15 years before making it to the Giants in 1979. He was around age 40, not "right out of college" as you put it.
My thoughts on the topic. Belichick was offered a job in 1980 by the Vikings where his friend Floyd Reese was already coaching. Belichick turned the offer down because he liked the east coast and the staff. If he had taken the job and left New York there would have been no Super Bowls for the Giants during the following decade. The defense would never have improved to championship caliber without Belichick. Parcells would have been fired within a few years by George Young (who wanted to bring in his friend Howard Schnellberger from Miami) and become a coordinator somewhere else or a college head coach. Today he would be a footnote in history, some guy who wasn't bad but didn't really win much either.