I think it has more to do with who we have as players than the players' ability to understand the scheme. Sure some of them might be slow learners, but it's hard for me to believe that the entire unit is unless the scheme is far too complicated, which I don't think Nolan's was. I do think because the scheme wasn't working that he needed to make changes to it ... and that he was slow to adjust. That's what I hold Nolan accountable for. He had one of the worst Cowboys' defenses of all time, and it's on him for not finding a way to make it better than that.
It's clear that we had personnel issues, though, and until we fix that, it's going to take some kind of miracle worker on defense to turn this into a good defense. Hopefully, whoever is hired next will be able to convince the Joneses in his interview that they have to attack the defense's deficiencies through the draft and with quality free agents.