So McVay and the Rams apparently had Tavon AND Watkins both struggling with the route tree?
As far as the Rams not re-signing Watkins I think it's as simple as McVay wanting Cooks from the beginning and all along. They needed a vet when they couldn't get Cooks last year so they were aggressive and didn't ignore the situation and got Watkins. He was a decent stop-gap and MUCH better than Britt and/or Quick but Watkins obviously has his warts.
He was on a contract year so the effort and motivation were there but I don't know that he was ever really a perfect fit for what McVay wanted.