Depends on how long of a time frame you're looking at, I guess, but if you just look within the first few years...
LSU didn't drop off after Saban.
Oregon didn't drop off after Bellotti or after Kelly.
Nebraska didn't really drop off after Osborne. Slightly, but there was nowhere to go but down after a 60-3 run.
Florida State didn't drop off after Bowden.
I don't think you can count Ohio State as dropping off after Tressell.
Wisconsin didn't drop off after Bielema or Andersen.
Stanford didn't drop off after Harbaugh.
Va Tech hasn't dropped off after Beamer.
so on and so forth