The extended German goal for the Battle of the Bulge was to reach Antwerpen and destroy the port so that the Western Allies could not use it to supply their troops, capture Western Allied supply dumps, destroy Westen Allied field air bases and generally destroy and cripple as many Western Allied field units as possible. The end goal of the offensive was to destroy the Western Allied ability to mount a huge offensive into Germany across the Rhine come spring 1945, not to drive the Western Allied out of France, which the Germans knew very well they were incapable of.

The Germans maintained garrisons in many if not most northern French ports as the Western Allies liberated France from German presence Summer and Autumn 1944. These towns and ports were often heavily fortified and the German troops in them had access to well-stocked supply depots of food and ammunition. The Western Allies considered it more important to liberate France than to deal with these holdouts, most of which would not surrender until Spring 1945. They were thus surrounded by small forces and left to be besieged as the Allied forces rushed across France. A side effect of this and the devastation to the French infrastructure in the fighting and the Allied bombardment of it during Summer 1944 to deny it to the Germans was that the Allied supply situation became critical Autumn and Winter 1944. It was no longer possible to supply a large wide-front offensive from the Mulberry mobile ports erected in Normandy over the long distances over the French countryside.

During Autumn and Winter 1944 the Western Allies had captured the large port of Antwerpen and had been clearing the port and the Scheldt estatuary from German troops and batteries in order to be able to use it to supply the troops in Belgium, southern Netherlands and northern France.

One of the reasons the German offensive surprised the Allies was that they correctly knew that the Germans lacked the fuel to conduct a major offensive. The Germans gambled that they would be able to capture Allied supply dumps and refuel from them. But even if that had worked out for them, which it did not, they did not have the ability to go anywhere beyond Antwerp.

A potential total success in the Battle of the Bulge for the Germans (which is very unlikely) means that the Allies has to re-fight to clear Antwerpen and the Scheldt estatuary and would perhaps delay their grand offensive across the Rhine by 3 months or so (it coming in June instead of March 1945), and meeting the Soviets at the Weser rather than the Elbe. At most, it could have bought the Germans another month, meaning they surrender in June 1945 instead of May, IMHO. By the time of the Battle of the Bulge, the Red Army was so much stronger than the Heer that even if the Allies did nothing, it could still defeat the Germans on their own.

The US had enough production capacity to support the Soviets, wage war in the Pacific, support the front in France and in Italy as well as supplying and equipping various free forces, the Chinese and the British and the Commonwealth. The limiting factor was not production capacity or economy but rather the ability to get the supplies to where they were needed.

Thank you so far for your excellent answers! Let me rephrase the inital part slighty: If the allies had suffered a severe defeat on the continent, with a loss of troops an material big enough to make them retreat to Britain, would there have been a back-up plan? Where the allies commited to a second invasion, was there a plan involving the atomic bomb on the western front? Or would they simply have waited for the russians to end this, accepting soviet domination of post-war europe ?