Oklahoma is about to have a Tom Osborne like drop off. End of an era.
They had a drop off when Switzer left, too. It's nothing new.
Oklahoma isn't currently at the pinnacle that Nebraska was at when Osborne left so maybe the drop off won't be as bad as you are expecting. Also, some schools can actually handle a succession plan correctly or they can just get plain lucky.
I don't think I know of a school that hasn't had a drop off after a very successful coach leaves a team
A few schools have managed to keep rolling but overall I think you are correct. Oklahoma, Nebraska, Ohio State, Penn State, Florida State, Oregon, Kansas State to name a few all dropped off for some period of time after a very strong HC Retired or was fired.