I have one problem with this all laying this off on coaching, the team is underachieving because of the coaching. Coaches do not block, run, pass, catch, make penalties or tackle.
Is this a good coaching staff, no, I do not think it is but the execution of the plays on offense and basic fundamentals are lacking on defense.
The single most overrated aspect of defensive football players is tackling. Johnson had the #1 D in yardage because of the tackling. He drafted players that could tackle and they gang tackled very well. If you look at the Patriots when they are at their best, they lead the league in limiting YAC.
I think some are fooling themselves into thinking this defense is a player or two away.