It's the idea that the grass is always greener, and sometimes it just isn't. I don't know of any elite coaches waiting in the wings to coach at Michigan. At best, you have people who MIGHT be successful. As far these Michigan teams compare to historic Michigan teams, Harbaugh has done well. I'm not fan of the guy personally, but as an OSU fan, it's better to have Michigan as relevant.
OSU had success because Meyer rebuilt the prestige and then Day was waiting in the wings to take over the program right away. Michigan will go through another down cycle hiring a new coach, with a new system, while we wait for "his guys" to play.