I know what you're saying - but IMO this is more a theory than what actually happens in practice.I should have probably re phrased that. Running back recruits usually don't want to be half worn down before they enter the draft. NFL teams will draft heavy workload guys because they know they won't resign them once their cheap rookie contract expires. If a running back wants that lucrative second contract, the more tread left on the tires, the better.
I don't know of NFL team that bases giving a RB a 2nd contract based on if they had 200 carries in college vs 600.
Big recent 2nd contract guys like Henry, Chubb, McCaffery, Cook, Ekeler, Taylor, Jones - all had huge College & NFL workloads.
If you're a special talent, you're going to want to the ball a lot, and you're going to get the ball a lot.
I agree with you that outside of those special "Top 10 in the NFL" type talents, NFL teams will just move on to a younger option because it's usually not hard to find similar talent for much cheaper. But even lower Tier RB's, I don't think their college workload is really taken into account when given 2nd contracts.