Humans are getting bigger, stronger, faster, etc. As the population of the world goes up, there is more of a talent pool to draw from, as well. Thing is, as QB humans get bigger stronger & faster, so do defenders.
Jim Brown was playing agains LBs my size. If he played today, he’d be a good starter, but not what you saw back in his day. Conversely, Barry would have run for 400 yards/game if he faced he same talent as Jim Brown.
Power backs have injury shortened careers. Bettis is one of the last one I can remember that didn’t have a super-short career. I can’t believe that Gore and Adrian Peterson have done, in terms of hard miles and longevity. Almost no players hold up to that sort of beating, these days…especiqally QBs. Time will tell, if Jackson holds up. He’s looking great, so far.
Even Cam Newton, who is bigger than the LBs, has slowed because of the beating he’s taking. There are exceptions.