My concern is that defensive coaches are notorious for not developing QB's into franchise level QB's. However, when a QB developes into a franchise QB it typically happens under an offensive head coach.
We have no franchise QB.
You also don't win superbowls anymore without one. Nor go deep in the playoffs.
Hiring a defensive head coach, no matter how good they are, feels like sticking a knife into the idea of getting a coach that can make us a superbowl contender in their tenure, however long that may be.