i think the problem with the debate here is there's just the ends of the spectrum, and nothing in the middle (context).
rarely is it just one thing, or another, with a group/organization/team that's comprised of multiple people....it's usually a combination of things.
could the coaches have done better? sure. said it already i don't think we did a very good job of making adjustments or reacting to changes made by other teams, especially the better teams we played....think 2nd half bama defense compared to 1st half bama defense.....as a glaring example.
did injuries hurt this defense? yes, we lost a couple of key players at key positions that really hurt...Hadden and Pili being out changed things. losing Arion Carter hurt too.
are the players we have/depth we have good enough to compete at the highest level? no, not across the board, and the dept isn't there. we have some very good players though, DL was good all year, Beasley had a good year...Mcdonald flashes.
anyway....we're not all that far removed from the debacle that was the last 15 years and the cupboard was bare for this staff. and you can tell with how they're recruiting on the defensive side of the ball. now we're not landing everyone we're going after, but they're going after difference makers at LB, CB, Safety and they've done pretty well on the DL....you can see it in how/who they're recruiting that this is a huge need.
so i think if you consider all the context of the situation, it's hard to just say 'its because of 'x', and if you change 'x', the result changes for the better'. it's a recipe and we need some new ingredients, we need some better ingredients and the chef needs to cook it better too.