Lets be honest, the fact is that finding a franchise QB is extremely rare. The fact we landed two in John Elway and Peyton Manning speaks volumes to be somewhat lucky. Lets look at the Cleveland Browns as an example that has literally gone decades between having that franchise signal caller. You had Bernie Kosar who was a very good QB and then Otto Graham who was the definition of the first what I would call modern NFL QB, now the Browns may be in line to move on from Baker Mayfield. I can point to many other franchises that have continued to struggle finding that magic behind center. But the fact is, finding that true franchise HOF guy is rare.
So that being said, it is a lot easier to have a consistently good offense with an average QB if you have a HC with a solid offensive mind. We can look no further than Broncos recent coaching to see this, our most successful HC were all from offensive backgrounds: Shanahan, Reeves, even Miller all were from the offensive side of the ball. I will use Shanahan as the classic example who was able to take a aging superstar in Elway and get him to buy into changing to a run dominated team that could set up the deep strikes with being able to have an effective running game. He took the pressure off Elway and put the emphasis on the o line and RBs to make the offense go, let Elway be the difference maker. But even after Elway left, the offense through Griese, Plummer, and Cutler was generally always considered tough. Belichek famously said Shanahan offenses were the toughest to prepare for. While Shanahan definitely had issues keeping a consistent defense, he still was generally had a team fighting for or making the playoffs even without Elway.
Lets look at another offensive HC in Andy Reid. His teams have consistently been solid offensively despite who was behind center. And once he got a guy like Mahoomes, he was able to get a SB victory. But his teams have always been competitive and much like Shanahan's tenure with Denver, Reid has been a guy who always seemed to get the best out of teams that maybe weren't the most talented offensively at QB.
But defenses win championships, and generally I say this is true, but it is much harder to have that elite defense that can carry a team to a championship. And while the last teams to do it that way (Broncos, Bucs, Ravens, Bears) ironically all had HC with an offensive background (Kubiak, Gruden, Billick, and Ditka) who understood they had elite defenses and they just had to make sure to play smart football on the offensive side and they could win a championship. But all those teams also prove that maintaining an elite defense year in and year out is very difficult in this league, mainly because of player turnover on the defensive side of the ball and injuries that can hurt a unit.
If the Broncos clearly had a franchise QB on the roster or if they were fore sure they were going to be able to land someone like Wilson or Rogers, then I would say a defensive coach makes a lot of sense. Since a franchise QB can make up for a lot of sins even with a average OC. Plenty of proof that a defensive HC with a franchise QB can be great in the league (see Belichek, Seifert, Landry) but in the case of Seifert and Landry, we saw what happens when he loses his top OC and franchise QBs.
The fact is until the Broncos truly land their next franchise QB, and that may be this year, maybe next, maybe 10 years from now. If we want to be a team that is consistently good, going with an offensive HC is a much better route to getting there than trying to stick with another defensive minded HC.