The Entente war economies by that late in the game were already deeply constructed on a framework of American raw materials that could be obtained will American credit. Those sources suddenly disappearing would be a massive shock to every supply chain that would utterly shatter Entente logistics; they might be able to hold a defense at higher costs in Europe,but you won't be able to fuel any offensives that would put the pressure on Europe. Germany will be in a great position in such a timeline to stabilize it's position, especially with the tensions in the east winding down as well, and can either dig in over the winter and dare the Entente to come at them with much lower stocks of shells and such, wind up for a knock-out punch on Italy, or even demobalize some men to help with spring planting to ease the food burdan.