If Manning and Brady had the Steelers defense it wouldn't be fair for the rest of the league.
I see variations of this argument all the time--i.e., it's not Roethlisberger, it's the defense.
The problem with it is that it's *always* been true that the great QBs have had tremendous players and schemes to help them. You point to Bradshaw, and that's undoubtedly true. But Staubach had tremendous talent. The Cowboys had dominant talent around Aikman. Elway didn't win until a dominant running game was built. Montana and Young had the greatest wideout in history, plus that era's genius offensive mind (Walsh).
The only exceptions to this are the Redskins of the 80s, with Gibbs...and your Patriots. They had great players, but not the megawatt, star-studded lineups of some of the other "dynasties"--which is huge testament to the quality of their coaching. But that said, do you believe Brady would be where he is, without BB, and without the excellence of the Patriots organization?
The point is, you can make this sort of argument about just about any QB. (For example: what if Roethlisberger was given a dominant offensive line, instead of what is arguably one of the 5 worst units in the league?)
FWIW I think Roethlisberger is still a long ways off from being compared to this sort of company. He's 28 I think--let's give it another 5 or 10 years and then see.
lillloyd