Writing Williams off is a stretch. He is a 2nd year guy and has shown he is a capable of being a backup IMO. He can pass Block and has a nose for the goal line.
The Rams problem is that as a starter is is very unproven and while he has some skills and some production he hasn't shown that he is ready to be a starter. The only reason he is starting is because Akers had his head up his butt. Ideally we would have a better plan B for a starter and Williams would be an adequate 2nd back.
Write him off? I say no way? Go out and get a real quality starting RB? I think that is the number 1 priority right now especially when there is a game changer out on the market.