The whole thing seems like a red herring. If GB were to hire some young up-and-comer who helped steer the defense to dominance and they won another SB next year, guess what? That young guy would probably get hired away sooner than a retread who's already failed as a HC. Any coordinator who is that successful on a title team is going to get a bigger job. See Philbin, Joe.
The biggest reason Dan Compers didn't even get a sniff at a HC job is because he's already been a HC and was not successful, which was why he went back to being a coordinator in the first place.
The premise of 'younger guy = stability' is not valid.