I thought the same thing about Georgia a few years ago. Then they fired Richt anyways.
For Michigan, getting back to respectability was incredibly important, and exactly what Harbaugh did. Now that they are respectable again they are watching Ohio State go through a nearly umprecedented era of success and dominance. They expect to take the throne from the Buckeyes, and in doing so go on to win titles. Rightly, or wrongly, that's the mindset. If Harbaugh fails to take down the evil empire yet again, I think they will strongly consider moving on because they want to get over the hump. The next guy may or may not be better than Harbaugh, but if Harbaugh has proven he can't beat Ohio State, at least the next guy hasn't proven he can't beat them.
I think they would be nuts to move on from him. But I think they would consider it. Even if they don't, pressure is going to be ramping up big time, and I could see Harbaugh deciding to jump back into the NFL next year anyways.