So, I think there is some wisdom to the argument that says "the eye test doesn't really count as much as it used to, because the game really has changed, and so NO defenses not named Alabama really dominate anymore, it's just a matter of holding teams down".
I think there's also wisdom to the argument that says "ok, fine, but we're still way behind average in efficiency numbers, so even if all defenses are down, Roofs is down even more".
I sort of come down somewhere in the middle. Roof isn't some X's and O's wizard. He's not going to do anything paradigm shifting, and he won't innovate. He's not going to scheme his way to a fantastic defense. That said, it seems pretty clear to me, both by the eyeball test, and by the "services" that Roof has improved the teams recruiting profile, particularly on defense. The improved recruiting on defense is starting to show in the 2 deep. Dudes like Alexander, and Gray, and Saint-Amour, and Adams are dudes that maybe GT doesn't get before Roof arrives. The dude CLEARLY loves GT, knows GT, can sell GT, and can close with GT. (there's also a positive feedback loop, where as better players choose GT, it becomes a reasonable choice for even better players)
I think it's entirely possible...frankly if you FORCED me to bet on it, I'd say probable, and lay my money down...that Roof recruits his way to a decent defense. I don't ever see you guys being a powerhouse defense, but a defense stocked with pretty good recruits, that have 3 or 4 or 5 years in the system under the same DC, with the same calls, and the same reads, you could have an above average D. Frankly, this year, I expect you to have at least an average D, and perhaps even better. I don't really see a huge dropoff in the following years, as the cupboard is relatively full.
Is it exciting? No.
Is it interesting...not particularly.
Is sound? Yeah.
Recruit good players, teach them a sound (if not particularly aggressive) scheme, give them a bajillion reps in it over their careers, and you'll put together a defense that won't embarrass you.
If you've got Johnson's offense, a defense that won't embarrass you will win a lot of games.