It's hardly a debate anymore.
Almost everyone agrees Tom Brady has surpassed Joe Montana as the greatest quarterback of all time. And the reason is obvious: Brady has won seven Super Bowls and Montana won "only" four. And Brady could win more. He's 44, he's coming off a Super Bowl victory and he's on the best team in the NFC. He has without a doubt the greatest resume of all time and it's not close.
But would Brady have the same resume had he played in Montana's era?
Football was a different sport in the 1980s. Defenses could tee off on quarterbacks and did routinely. There was no "strike zone" when hitting quarterbacks -- defensive players could hit them anywhere they wanted and as hard as they could. Plus, defenses could hold wide receivers all the way down the field, which means quarterbacks had to hold the ball longer in the '80s and take more punishment.
Montana took an absolute beating and injured his back as a result. So he couldn't play into his '40s -- no one could back then. Injuries caught up to players. Terry Bradshaw won four Super Bowls in his '20s, got injured and retired at 35.
It's likely Brady would have suffered a similar fate during that era.
I doubt Brady would have had the same longevity had he played in the '80s. Brady cannot move -- he's a statue compared to Montana. That's why Brady suffered a gruesome knee injury when he was 31. Afterward, the NFL changed its rules to protect quarterbacks when they're in the pocket.
The NFL never changed any rules to protect Montana.
Had Brady played in the '80s, it's possible he would have won those three Super Bowls he won early in his career. But then he would have injured his knee, and probably faded away the way most quarterbacks in their 30s did back then.
Don't get me wrong -- Brady still would have been great. Just not as great as Montana.