How can you say on the one hand that coaches don't matter, and then on the other hand call someone a "bad" coach?Bad head coaches who won a Superbowl:
Weeb Ewbank
Don McCafferty
Mike Ditka
George Seifert
Barry Switzer
Brian Billick
http://en.wikipedia.org/wiki/List_of_Super_Bowl_head_coaches
All these guys won a SB, but failed hard afterward. Heck, Seifert eventually went nearly WINLESS with the Panthers...
A case could even be made for Jon Gruden.
If coaches didn't matter, you'd have no way of knowing who was good and bad. You'd have only "neutral" coaches who either had good quarterbacks or did not.