Him not getting a head coaching job has nothing to do with his defence. A lot more traits to be a head coach rather than just coaching a good unit. (Age, leadership etc- the list goes on)
He has coached his defence to 3 Super Bowl wins, and maybe had the greatest display ever against the 2007 pats. And you're focusing on a few bad playoff games?
Should we go over McDermott's bad playoff games? I don't think we can over the good ones, because they there aren't many