If the Germans continue the fight their going to have to face the wrath of a fully mobilized America, a large powerful nation with massive reserves of men that the Germans in 1918 don't have. In fact the Germans are running low on everything the civilian population is straving and the Army is running out men, weapons, and ammuniton as well as the will to fight. The Germans are in for a far worse time if they fight on, so ok lets say they fight on flash forward to 1919 American men and guns are pouring in by the hundreds of thousands and they've begun to replace tired and worn our French and British units, at this point the Germans would be running low on ammunition, food, and soldiers and the home front would be racked by simmering revolts.
Now its late 1919 and the Germans have been fully pushed from France and Belgium, at a great allied cost but the Americans take the brunt of the fighting and are able to replace any loses with ease while the Germans now struggle to find enough men to man defenses on the German border, the Germans at this point are running dangerously low on artillery and other heavy weapons seeing as most of it was lost during the retreat through France and Belgium, the East is awash with revolution so the Germans are surrounded on both sides to the West the Americans backed by French and British forces are preparing themselves to strike into Germany proper and to the East the Poles, Russians, and other various waring factions are doing there thing and in Germany its self the people are starving in the streets and on the verge of revolt.
The Germans are going to have to surrender, they have no choice they can't stop an allied advance into the Empire due to the lack of men, equipment, and supplies. The people don't have the will or ability to fight on, their broken and beaten. This post war Germany will be much harder hit and suffer even worse punishment at the hands of the Entetent this time around.
It would be intersting to see triumphant American, French, and British troops marching through Berlin in early 1920 from the eyes of a young German. I know its kinda ASB but I still think it would have the potential to be a telling tale.