If by some miracle the Germans do manage to hold on the line and defeat all allied attacks in 1918, then the Allies are coming again in 1919. In fact, they expected something of the sort to happen and were preparing for it. Expect thousands of tanks and aircraft, tens of thousands of artillery pieces, millions of men and paratroopers used for the first time. Germany would actually be worse off politically, as the population would collapse at home, maybe even letting several of the communist revolutions take hold, as the troops that defeated them OTL are still at the front ITTL. There is no stabbed in the back myth, as Germany is thoroughly defeated. The peace at Versailles might even be harsher, as the Allies had to bleed that much more before forcing a peace.