A head coach is an inspirational leader and evaluator of talent. He is also charged with the duty to manage the game, gauge the players temperament and be a figure head who the players listen to and respect. I doubt there is another candidate out there who fits that role better than McD in the eyes of the people it matters to most - the players and the owner. Every fan clamoring for a new head coach based on mistakes made by all different individuals (McD, Bass, Johnson with PI, etc.) ranks at the bottom of the barrel compared to those who want him to remain (Josh Allen, T Pegs and Beane).
BUT - He does need a like minded full time DC (hopefully someone who shares his attraction to fast LB's, Tweener safeties and fierce pass rush to force QB errors). McD has enough on his plate with player morale, game day decisions, etc. He should work in conjunction with a DC who shares his vision of the defense and concentrate solely on head coach responsibilities.