Excellent point. Teams play less aggressive because they have to play so much.
Additionally, the players today are much better athletically versus 50 years ago, they train year round, the best RBs in the NFL right now are arguably Jonathan Taylor and Derrick Henry, both are huge and fast, Henry is 240-250 lbs. in 1975, he would qualify as a lineman, players today are bigger, stronger and faster, plus they play more games, combine all that and it equals more injuries....which equals poor technique because of lack of practice.