The issue with hiring a defensive guy is the instability it can present to the offense and potentially your QB. Offensive guys tend to be the guys that get hired away for HC opportunities.
I agree, I would lean very strong to an offensive guy in today's landscape, especially with a young developing QB. Can't make a blanket statement but the QB/coach combo is just so crucial in today's NFL.
Hiring a good D oriented coach sounds nice, but then the OC hire becomes even more important. Problem is, you're left with basically 2 options.
1. Bad OC hire, likely sets the young QB back, and have to hire a new OC soon.
2. Good OC hire, gets interest around the league, hired as a HC in 1-2 years. Need a new OC soon.
I guess the goal is option 3..... decent but not good enough to yet yanked away for a HC job? Not ideal.
You might get lucky with a BB/McDaniels situation that works for many years (til now potentially), but that's not the norm.