This is the mistake that too many teams (including the Rams) make. The odds are never in favor of a RB lasting much beyond 5 or 6 good years. But, teams will pay 5th year star RBs big bucks and more often than not it becomes a waste of precious cap space.
Obviously there are exceptions to the rule. But there's little way of predicting it; and betting the house on finding the exception is not a good way to go. That said, when it comes to RBs, it is their style of play that can lesson or extend their playing careers. If they are all about shiftiness, quickness and speed, they are likely to fall off the map more quickly. If they are about physical play, North-South type runners, they can play longer effectively provided they don't have mounting injuries.
Sony Michel is a case in point. He is one of those who has a better chance of longevity. On the other hand, it would be foolish to give him a big expensive contract at this point in his career. He's got a good amount of mileage on that body, and especially this year. Is he worth perhaps $3mil per year? Yes. $5 or 6 mil per year? No.
I'll be curious to see if teams line up for Sony. I feel confident the Rams will let his agent know they'd like him to stay. But they'll also make their pay parameters clear.