I'm worried this question may verge on a what if, but I'm going to ask it any ways. I know that roughly speaking France, England and Germany started with with their primary armies and reserves, and during the course of the war as these got depleted they drafted, recruited and generally kept finding new sources of soldiers. How tapped out were these countries? If America hadn't entered and the war kept going several more years, what were the plans on both sides to find new soldiers to replace losses? Theoretically could the sides continue to field armies of the same size?
I know there were all sorts of other things going on, supply shortages from the blockade, changes in tactics, technology and everything else, but I am only interested in manpower for this question.
Germany was pretty well tapped out by 1918, the shift of c. 500 000 men from the Eastern Front having only temporarily mitigated a manpower crisis that had plagued the German Army since the Somme. Britain made it through the initial manpower crisis of 1918 just fine, but was looking at difficult times ahead in 1919; France's position was worse, the losses from 1914-16 having added up. The SOP on both sides at this point was to replace manpower with firepower: German divisions were about 6600 strong, a third their original size, but with over 200 machine guns. However, these divisions burned out fast, and I can imagine ammo. supply was a chronic problem.
Combine this with the fact that without direct Government-to-Government loans from the United States, the Allies could not have easily financed the war after June 1917, American entry was crucial. The AEF didn't really see major fighting until the Meuse-Argonne Offensive, an their training and organization left much to be desired, but their presence sealed the deal essentially for the Allies.