Maybe Casteel is a great guy and he has had success as a coordinator. I have no idea how or if he recruits, but he is a younger guy that should (should) be able to identify with recruits.
However, a few things:
1) If we are to fire the second DC in three (!) years, I think it is time to bring in an outsider in that is given the authority to broom the staff on that side of the ball who have remained a constant through this disaster. I don't trust RR to "get the gang back together" since I am sure Tall* and Gibson would then stay on board. It sort of feels like we are at the table doubling down on the same bad bet.
2) I actually do believe that scheme does matter. I am done with the 3-3-5 nonsense and this would be a result of a Casteel hire. I am also tired of hearing my head coach say we go between the 4-3/3-4/3-3-5, if we have three schemes, we have none. There are many top defenses in this league, they led by DCs who are paid many dollars to coach against teams in this league, so run a defense built for this league, not one that is made to stop our offense. On the flip side, we have already seen how defenses line up to stop our own offense from scoring (not gaining yards), let's just copy that when Nebraska shows up next year since they are the only team who looks like us (Illinois may get there with the new QB, I don't know).
The only way someone good will take a chance on this if GERG is gone is if they have full run, the dump truck of cash is backed up and maybe a multi-year contract is signed. Otherwise, the track record looks poor from a longevity standpoint which makes hiring more difficult.
*For the Tall supporters, I am not so sure Tall made BG a first rounder. I think BG's uncanny explosiveness/strength, Barwis and his ability to destroy in 4-3 scheme that fit his talents appropriately (e.g. like Griffin from USC, not dropping into the flats to cover TEs) made him into a first rounder.