Brady has won more Super Bowls and Conference championships and playoff games than anyone. His career has lasted longer, and while he was mostly just a 'game manager' at first, he has long since become recognized as among the league's elites. He's not necessarily the best EVERY year, but no one can match the list of accomplishments. And he's done it with a middling set of receiving weapons. Sure, Gronk, Edelman, Welker, even Moss for a time, are all good players, but compare his receiving weapons with most elite QBs, and Brady's looks weaker than most.
As for the 70s and 80s, it was a very different game back then. If Brady were to time travel back to the 70s, and 80s, and try to play in the NFL, I agree, he would not be a great fit. The game was more run oriented, the offenses were fairly unsophisticated.
The big advantage for Brady is that pass rush pressure was WAAAAAAYYYYY slower back then. If playing under modern rules, with his offense and players he practiced with, he would absolutely SHRED the Steel Curtain or the 85 Bears. It would be ugly.
But that's because back then, defensive backs could mug receivers, quarterbacks could be crushed, and receivers generally didn't run precise routes. Not many teams were running the kind of anticipatory offenses Brady thrives in. Without that, those defenses simply didn't have the speed and athleticism to match up.
Now, if playing under 1970s rules, Brady's offense simply wouldn't work. It's predicated on choice routes and free releases and anticipation that a receiver is going to be where he's supposed to be when he's supposed to be there. You couldn't count on that in the 70s.