I'd like to see the evidence to back up what you say to be true particularly as it relates to the last 25-30 years because before that just isn't applicable.
There simply aren't that many guys ( a mere handful) who have gone from NFL only coach to college to back up what you claim to be true to make it a meaningful sample.
The fact that there are so few to make the move when there are so many failures/firings at the pro level In other words( the pros create 3-4 former head coaches every year that would be ripe for the move) certainly indicates a recognition of the vast differences in the two games and it certainly isn't about money anymore.
All in all even with the silly ass speculation (granted there is little else hopeful right now) to bring up names like Mike McCarthy, Bill Cohwer, Mike Shanahan, Jon Gruden, and my favorite, Tom Couglin ( who folks have been bringing up for years because he was a coach in college 20+ years ago) etc is fantasy at it's highest level.