Just because you have beef with Cook does not make this true.
Every year the cap goes up. Every year contracts go up. Numbers go up. That is a fact. It's why there were a number of "lesser" QBs making more than Josh over the past few years.
Barkley reset the RB market with a $20M+ contract. Whether you believe he is close to Barkley or not does not matter. No one is saying he should be paid as much as Barkley. BUT, if Cook does have another year like last year, then he will be known as an integral part of the best Offense in football. If he hits the market, other teams will not care whether he is a 2-down back. With 2 seasons in a row like that, he will absolutely get $15-17M on the open market next offseason.
His tag number will be $13-14M, and if that's the case, Beane will tag him. Heck, he might tag him anyways since it's essentially a 5th year option.