Pete Carroll started taking down his own empire by pushing away Norm Chow. Even though Chow's character may have been partially responsible, Pete was the emperor and his ego pretty much played the major part of Chow's departure. The Texas game finally showed it. USC never returned to the NC games since then.
However, the idiotic decisions on the new coaches after PC were the reasons that we are at the rock bottom right now. Kiffin, Sark, and Claydo combined to send us to the bottomless abyss as of today.
A good coach, if we can find it, can turn the tide sooner than we can imagine. We're pretty much at the same situation as the end of the Paul Hackett era. It took only 1 full season for PC to turn around.
However, the big if is, we find the right coach. Many potentially "supposedly great coaches" didn't end up what they were expected: Kelly among one of them; Chris Peterson, Jim Harbaugh, you name it. It's hard!!