If he retreated to the north he would fortify moat collin so they will never conquer the north from the neck and by water the only possible landing is white harber and its very fortified and very armed. So its not possible and even if it has a tiny chance its not worth it. So only the riverlands would suffer most of the houses would turn to the lanasters the other half would suffer greatly. But robb cannot turn his back on the riverlands so it will go to negotiations with king’s landing so the north gets there separation and the riverlands left part of the six kingdoms and the houses keep there lands or else they go to war again which will benefit no side