I often wonder about using a time machine and swapping QBs of different eras to see how they would compete.
Just a hunch, but I have a feeling Brady would wither away and die an early football death if he played in a less QB-friendly era. I think some of the old time greats would put up similar yards, TD:INT ratios, and passer ratings to the QBs of today.
Bills example: Tyrod is the highest rated (passer rating) QB in Bills history, isn't he? There's no way in hell you can convince me that he's a better QB than Kelly.