Would a Horrible Performance by Detroit Defense in the playoffs change your mind on Aaron Glenn ?
Not really, offenses are attacking that particular scheme at a high rate. Glenn adjusted in a way that Allen didn't by blitzing on early downs, and then blitzing/playing man w/out fear on passing downs. Anything to make sure the team went into 2nd or 3rd and long so they could attack. As noted by Underhill, Allen refused to blitz consistently even when he found late in seasons that it worked. The following season he would just go back to playing more coverage on the back end and it resulting in us getting exposed as team started passing less and running more.
We stopped playing single high for more of a 2-High system w/ some tampa two elements and we got gashed in the run.
So in other words, Glenn will probably have a bad game....just like Gregg Williams defenses had every now and again, as a bad game for them is one where they don't force turnovers (giving up yards is normal). In that game it'll be up to the offense to score a lot of points and Detroits offense is built to do that so they'll be fine, including Glenn.
The obvious bad thing about defensive HC's or HC's like Rizzi is that if you get a good OC, in 2-3 years people will be calling for him to be a HC. So you're constantly having to cycle people and have a plan. You get an OC as a HC and not many DC's get the call to be HC's. So you can keep them around longer and it's more stable. So though I'd like Glenn... I'm actually leaning towards McCarthy because he's a built in playcaller.