Whaley/Nix deserve the credit for last year's draft, not Pettine.
My take on Pettine's departure:
Our defense will continue to improve next year, regardless of who the new DC is. We are simply loaded defensively, aside from LBs which will surely be addressed this off season. Also, I suspect we will be ranked higher against the run. Remember how often we were trappled on last year by almost every RB?
So Pettine, thanks for the sacks and turnovers. Good luck in Cleveland.