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

You've hit on something many of us have also realized. The "average" RB of course only enjoys a few seasons in the league, but this shorter "shelf life" has extended to the upper echelon as well. Adrian Peterson and Frank Gore are the exceptions, it seems, to a rule that for most RBs says that the moment you hit 30, you're done and you're not going to get another contract.

Part of this is indeed because the sport has changed, thanks to the way the rules are interpreted and the way it's coached. Every level of football is about the passing game now, with the running game de-emphasized. Still important, mind you, but it's not the main thrust of moving the football anymore. At every level now, you pass to set up the run later on in the game. Running styles like Derrick Henry's are the exception, not the rule. And not even he figures to still garner much interest by the time he gets in his 30s.