This is where I disagree. The Patriots Dynasty as it happened would not of happened without Free Agency. It allowed them to reload the roster quickly where in the old days those players would eventually retire and backfilling solely by the draft would have been unlikely. As long as the QB stays the same the rest of the parts can change.
The relationship between how big a role the coach plays and how big a role the QB plays is forever up for debate. Andy didn't do what he's doing until Mahomes came along. That was after a very long career. Same with Bill and it most certainly ended as soon as Brady left. We know these guys are great coaches, but they don't seem to hit this legendary status until they have the QB. So I think it's likely a bit more common than we would think. That QB is creating an environment that is fairly ideal for these HC's which allows them stability, tenure, growth. If they're good they become better as a result.
I mean, maybe, but we are seeing something that hasn't occurred before. So you can either take on the perspective that it's random or you can take on the perspective that its not random. It's logical to me why this is occurring after a prolonged run by the Patriots. I expect it will keep occurring (extreme or prolonged dynasties). We will see over the coming years. Appreciate your post.