If Michigan fires Harbaugh, they're insane.
I've argued for awhile that Michigan is the most overrated CFB job. It's recruiting base has disappeared as American economic might has moved both southward and to the coasts. It has to compete at a severe disadvantage with schools like Ohio State and Penn State, which both have much better natural recruiting footprints. Michigan has the prestige and resources to lure recruits from Florida, Texas, and California, but it's a lot easier when you can grab 8 - 12 recruits from your home state / region; and not have to get all of them from 1000+ miles away.
Harbaugh is 38-13. That's pretty good. Harbaugh gives them their best chance at a national championship in the next decade.