I agree with him being the wrong appointment and it being an easy job that he badly messed up, but you're putting way too much at his door to say we are only clearing up the damage he caused. The damage was done by a combination of Coyle, Rosler and MacKay over a 2 year spell and you could even say Caldwell, Joyce and Barrow deserve blame too for the set back of relegating us to L1 for a second time.
When Coyle left after six months we'd massively under performed but we still had a good team, had some good new signings along with the duds (Perch, Powell, McClean, McCann and initially Barnett) and 3 and a half years of parachute payments it wasn't a crisis at that point. So he can't be blamed for what happened after he left, he was just the first in the line of failures. Rosler probaly did more damage than Coyle due to blowing so much more money and signing all duds and dragging us into a relegation battle and destroying our confidence leaving us in a mess that the hopless MacKay had no clue how to clear up.
I don't like having to defend Coyle as i was as against his appointment and for his sacking as anyone but i understood why Whelan wanted him even if if i disagreed myself it wasnt completly crazy or totally void of logic. Also i don't think we can throw excessive blame Coyle for the disasterous 4 years or so spell with 2 relegations when he was only in change for few months. He may have set the ball rolling but others had more than enough of a chance to stop it and ended up pushing it down the hill even further and faster.
You could even make a case Martinez set the ball rolling on our decline by relegating us in the first place. I'm just glad we've finally back on the right track with Cook.