Kelly told us all "year one" that defense was about first getting the player's "work volume" up to more acceptable levels. As said by RDU, we were miles away from acceptable ability to endure X-number of snaps then [remember the Kelly/Nix conversations about maybe getting only twenty snaps a game because of conditioning], and even though we are now in much better condition, we cannot go all game long [like O-Lines often do] with the kind of exertion/violence D-Lines need to exert every play. Thus: rotations. We do not have eight great D-linemen and especially four great DTs. Teams all over the country who don't have recruiting advantages over Texas, Alabama, USC, OSU, etc have found that DTs are the most rare and precious of large ugly desirables.
And there is a second issue: 21st century defense is not about doing anything [line-up-wise] which is similar every time. Unless you are completely uber-studs, offenses are too good and will find a way to pick you apart. So flexibility and rapid pre-snap adjustments and disguise are vital. People like BGIF and Whiskey have pointed out many times on this site that we line up "four-down" almost as much as we line-up "three-down". When I'm thinking about pass-rushing, the front of Lynch, Nix, Tuitt, and SHEMBO is about as good as it gets as far as I'm concerned. AND, if they're expecting Shembo, and he unexpectedly drops but here comes Te'o or Slaughter or whomever as a surprise, well that's pretty good too. Diaco's pretty smart, methinks. I believe that all that has been holding him back is that he is not yet confident in his LB-in-space play, plus the Coach wants him to always prevent the big play. The LBs are right there I believe, and if we get QB play, Kelly will have confidence in the 40-point-per-game offense again, and will be happier with a little more riverboat gambling by Diaco.
That's my story and I'm sticking with it.