Why is there no more longevity with today’s RB’s?

My two cents:

1. Players and teams are wary of concussions now. So early retirements or other teams don't want to touch them if cut.
2. Because of how strict the helmet to helmet is now...defenders are going for the knees, legs, ankles. More season/career ending injuries there.
3. It's become a pass-happy game. So if you are not a running back who catches out of the back field...there is not as big of a market for you. Therefore teams can go with a cheaper option.