Let's look at the prolific New England Patriots. They are a good case study because 1) they replaced the franchise QB with a sixth round draft pick and went on to win a lot of championships, and 2) they are about to replace the franchise QB with ... someone.
Bill Belichick protected his 'system QB' with a defense that allowed him to win without winning. TB12 began his historic run with a controversial call on a fumble/incomplete pass. He marched the team down the field so that a kicker could win the game. That kicker won a few championships for New England. At Brady's peak, the team surrounded him with some great tools and won some more championships. Now, Coach is focusing on defense again as the team prepares to bring on a new QB.
If Bill cannot win without Tom, I would have to concede that a stud QB is the foundation of a winning franchise. If Coach can continue to win with Tom's replacement, then I would reluctantly allow that these stud QBs are overrated.
It's all situational, but I am hoping that the Patriots find a way to win after TB12 moves on.