I don't like it, but I think you're right, save for the conclusion, and I'm not saying even that is right or wrong. I can certainly appreciate the sunk cost analysis. At 3 million dollar after his bonus, nothing is preventing another team from renting his services for a year, so a trade may yet be viable and this team will have to decide whether or not that $3M can be used elsewhere to greater affect.
It may be captain obvious to say so, but kid keeps playing himself out of roles with McVay who doesn't seem to trust him with anything other than a hand off at this point, which I can personally appreciate. Forcing the ball to him on the premise that he's capable of playing up to his salary/draft expectations has absolutely been detrimental to the offense in the past.
Is he worth $3M to us as a backup running back? I personally don't think he has the vision to take the hand offs on a consistent basis if something were to happen to our starter. He is a legitimate option addition to Gurley in the backfield, but if it's a sweep where he is lined up as a slot receiver, that's also a tell, which could be setting up the situation where another player would sell that option better as a legitimate threat to catch the ball (which has already been stated in this thread, just giving credit where it's due).
In the end, people are going to continue to lament the cost benefit aspect of his services, but the real 500 pound gorilla in the room is the "he has to be good because I bought his jersey" crowd feeding the cycle by insisting that every coordinator he's played for can't "get him the ball in space", but the tail wagging that dog is comprised of a dead horse to beat upon until it's not by the "yeah, but he can't catch the freaking football" crowd.