The overall way you are looking at things is perfectly fine. It just appears to me to be a little outdated in a few areas.
The first area is in the valuation of a DT and specifically a pass rushing DT. 50 years ago the value of a DT was probably in line with what you state here. But in the last 20 years, the game has changed a lot. Offenses are much more high scoring and dynamic (thanks Mike Martz). Rules have changed to further bolster that trend. It’s now a passing league. Defenses have adjusted to that reality. So now,
ANY defender, at ANY position that is great at rushing the passer is the highest valued commodity. The salaries that NFL executives are handing out to pass rushers - whether DE, OLB, or DT – verify this.
Secondly, I think you underestimate the value of pass rushing in Wade’s D. Yes, it’s been said that his defense is designed for Dbacks to shine. However, Wade himself has said that pass rushing is critical to his defense. His is no different than any other modern D in that regard. In fact, the primary reason he likes the 3-4 is because it affords him the ability to generate a better pass rush using the element of surprise. Phillips is about as aggressive with rushing the passer as Greg Williams is. So an elite pass rusher in his defense is just as valuable as one in any other D.
You also state you can get a good DT for $7 - $8 mil. Five years ago, that statement is probably true. Today a good DT on the open market is gonna cost you $10 mil. And if the guy is elite at rushing the passer, he’s going to get elite pass rushing money no matter the position.