he is making a ton of money at Michigan. But, more importantly is the head hancho in charge at Michigan right now, which doesn't have a price tag on it. He could walk into a classroom during a lecture and have every student flock to him like he was one of the Beatles getting off the plane way back in the day. Then look at the professor and sau "they all get A's today" and no one would question it.
That doesn't happen in the NFL. He is a legend right now at Michigan, and given his history and his dad's history there I would be shocked to see him leave before delivering at least a playoff appearance.