There's no way to consider this question without including the effects of the Central Powers, Germany and the Austro Hungarian Empire, winning the war. Russia had already made a separate peace which included the loss of tremendous amounts of their territory. It's unlikely the Bolsheviks could have maintained power and the Soviet Union would not exist.
I seems highly implausible that the Central Powers could have won if America had remained neutral; it would perhaps have taken them another year to lose, but there was really no path to victory for them on the Western Front once the line stabilised. Having failed to flank the British and French armies during the 'Race to the Sea', the Germans simply had no answer to the question 'how can we defeat France' after October 1914; their only hope was to outlast the French and British, which their geographic position and the strength of the Royal Navy rendered impossible.
By the time that the AEF arrived in France in significant numbers, in the summer of 1918, German troops were routinely carrying out trench raids with the objective of stealing food from the opposing British and French armies. The food, fuel and other supplies situation in Germany itself was even worse, with the military getting first priority for supplies.
Germany couldn't win. The only question was how quickly they would lose, and how painful the terms would be.
The biggest effects of US involvement in WWI were domestic and diplomatic; their military contribution was negligible. Only about a million Americans saw action on the Western Front, compared to over eight million French and two million British troops active in-theatre at the time. Doubtless their contribution was important in saving huge numbers of lives and shortening the war by at least several months. But the outcome of the war was not in doubt by the time they arrived - nor even a year earlier, when America formally declared war.