Thanks for agreeing with me, because what you wrote is an extension of my point. If professionals can't predict with any certainty who is and who isn't going to be successful, than who would imagine that some group of random fans could.
The fact that one defensive coordinator was unsuccessful doesn't mean all defensive coordinators are going to be unsuccessful. But fans, who have very superficial views of the whole process, might arrive at that conclusion.
Ash's lack of success might be caused in large part by a lot of factors out of his control. Only Schiano has had any success there and I have always felt that his success was based on lies. He left there, in part, because his lies about where the program was going were starting to become obvious.