I like Cook. I like how he's played the last two years.
I also think the best use of him as a resource right now is to trade him at the top of his value. It's a win to use a late 2nd on him, get 3 very good years of production and then recoup that pick and use it to get better where we have bigger needs. I am comfortable with a Davis/Johnson/rookie backfield and continuing that cycle.
The main reason not to pay a RB is the likelihood of injury at the position. The more carries they get throughout their career, the less likely they are to stay healthy. Additionally, they are more susceptible to getting a lower body injury that they never fully come back from. I am not comfortable ever giving a RB a 2nd contract and tying money up in the position on a long-term, high guarantee contract.