coach to bring in another coach will always result in a lesser record for the next 3 to 5 years.
While this may be the exception to your rule look at what happened to tOSU when Jim Tressel was let go, yes he was let go for other reasons than not winning and the Fickel temp season wasnt exactly great, but lo and behold Urban Meyer comes in and guess what tOSU is looking pretty good.
I dont think we will find the next Urban Meyer but saying that teams ALWAYS do worse is hyperbole.