That's great but the group of head coaches who I highlighted are all guaranteed HOF'ers were all defensive coordinators who made an impact. It's not always OCs regardless of the NFL's inexorable march to make playing offense easier and easier.
Fangio and Flores might stink as head coaches and may never get another shot but it's not guaranteed that OCs are going to be the flavor of the week going forward.
The only guaranteed HOFer in that group is Billichick. For coaches it's extremely hard to get an HOF opportunity. Mike Holmgren is still waiting despite having a better career win percentage, better postseason record and more Super Bowl appearances than both Carroll and Tomlin.
I'm not saying defensive minds can't be good HC's, there are many good defensive oriented HC's. If we look at recent Super Bowl trends, The last 4/5 Super bowls have been won by more offensive oriented HC's. The last four of the Superbowls have had offensive minded HC's on both of the teams.
This season, if we look at the playoff picture 8/12 of the teams competing this week were headed by offensive minds. Defensive oriented HC's can be and are also successful, but the skew has definitely been towards offensive oriented guys in recent years.
Now, at the end of the day the main thing is: how good of a manager is the guy? This is the real kicker here.
Managing big personalities, crafting culture and surrounding yourself with the right staff is extremely important. If you're good at these aspects and have good situational awareness, you don't even need to be a brilliant play caller. It isn't uncommon to see HC's not call offense or defense.
You need to have the vision, culture and people management skills above all else. This is why guys such as Josh McDaniels are great OC's, but AWFUL at HC. In the case of McDaniels, he doesn't have any of those skills.