You can bring in the greatest defensive coordinator who ever lived and it would make no difference. It is the players. Too many times the players don't know their responsibility, are just freelancing, or are making business decision tackles. Too many players are just weak both mentally and physically.
Too many players cannot even fill their gaps. Too many players do not even know if they are playing man or zone. A defensive coordinator can't even dumb down a system for many of the current players. Even the perfect defensive call gets washed out because some knucklehead is afraid to tackle and is ankle grabbing. As a GM of another team, ask yourself who would you try to trade for on the Cowboys' defense.
The only players worth keeping are (in order): Diggs, LVE, Gregory, Gallimore, Aldon Smith, Brown, Lewis(serviceable), and Lawrence (begrudgingly since he is way overpaid for his little production).
Possible upside players worth keeping: Anae, Carter, Hamilton, Bernard, and Reggie Robinson (McClay's pet cat being the only reason).
Good luck trying to build a defense around those players.