Jason Kirk at SBNation has a good article on why the Harbaugh's, Chip Kelly, etc. would prefer not to go back to coaching in college again once they have established themselves in the NFL.

While TTUN could throw a lot of money at potential HC candidates, I think the article reasonably makes the argument that poaching Jim Harbaugh from SF (or whatever NFL team trades for him) is little more than a pipe dream.

It's rare that a successful NFL coach returns to the college ranks. Usually it's the ones who wash out and try to go back to their former arena of success. (See: Butch Davis, Dennis Erickson, Steve Spurrier, Greg Schiano, Lane Kiffin, Bobby Petrino, etc.). That doesn't mean these failed NFL coaches are a bad choice. Some have succeeded upon returning to coach the kids, but some have not.

However, successful NFL coaches will prefer to look to other NFL opportunities before returning to the college ranks. And make no mistake, with 3 straight playoff appearances, an NFL Coach of the Year award, and a 23-8-1 record the past 2 seasons, Jim Harbaugh is a successful NFL coach. (Even if he is a bit abrasive in his personality.) He is still in demand, and will have plenty of suitors in the big leagues that can pay much more than TTUN should his tenure in SF close at the end of this year.

Whether or not TTUN gets its man in Jim Harbaugh, only time will tell. I'm certainly not betting the farm that it will happen, though.