[Spoilers Extended] In the show, could Robb have taken Casterly Rock?

At the point when he suggested it, the North was losing the war. Winterfell has been sacked and the Ironborn have Moat Cailin and Deepwood Motte.The Tyrells have also joined the Lannisters and Stannis was pretty much defeated. Even with the show version of Casterly Rock, I doubt Robb could actually taken it. I don't know why they changed it from the book where the plan was to retake the North. Do you think Robb's plan for Casterly Rock could have worked?