You are eliminating 31 years of NFL history based on a condition that didn't exist. Leather helmets went away in 1949
Minimal athleticism?
Another false premise.......unless you want to convince me that guys like Bob Hayes, Jim Brown, and Gale Sayers had "minimal athleticism". Some of Olympic Gold medalist Bob Hayes track records have yet to be broken.
It would be more correct to say that 1980 to the present was the period of time in which rules were changed to protect players from getting hurt, thus, allowing them to utilize their superior athleticism.