Using German troops from the former eastern front to win in Italy and the Balkans would theoretically allow the austrians and Bulgarians transfer hundreds of thousands of troops to the western front as well as the German troops. Knocking Italy and Greece out of the war would also be a huge blow to British and French morale and could allow the central powers to start a new front in southeastern France.
I suspect that with the right cards Germany might prolong the war into 1920, but by 1918 is it within Germany's interest to do that, or is it better to get it over with? What if prolonging the war turns the US to unconditional surrender?