I'd argue Harbaugh effectively left San Fran for Michigan. He wanted the GM powers and they wouldn't give it to him. So they mutually parted ways and 2 days later he had the Michigan job.He isn't willingly leaving an NFL job for a college job. The only ones in recent memory (that I can think of) who did that were Saban leaving Dolphins for Alabama and Petrino leaving Falcons for Arkansas. I'd take Petrino out of this since he was proven to be a head case. Rhule ain't Saban and PSU ain't Bama (in terms of boosters pressure).
Now all that said--I just looked up Rhule's contract and he's locked in for $60M through 2027 at $8.6M per year. So yeah you're right, he's probably not an option.