In mid-1917, the French army mutinied and was likely unable to resist a concentrated German offensive if it came; the British pushed on a major offensive in Flanders in part to prevent the Germans from realizing this. In 1918 (by which time the French army had recovered from mutinies), the Germans tried their Spring Offensive. And it failed for pretty much the same reason their original push in 1914 failed: their logistic systems just couldn't sustain the offensive long enough to prevent Allied reinforcement.
Even without US involvement, France, Britain, and Germany were effectively all devoid of manpower. Once the Spring Offensive failed, there really wasn't any ability on any side to carry on offensive operations, at least to the point of producing a clear end to the war. So the first country to fall apart from the stalemate would be the one to lose it all. And Germany certainly seems the strongest contender for being the first to fall. It was having problems keeping even its soldiers well-fed, and the Spring Offensive produced a very indefensible salient. Germany's hopes of victory died at the Second Battle of the Marne, and they probably would have died there even had the Americans not started streaming to the front lines.
Agreed that the French mutinies presented a unique opportunity for the Germans but it's hard to predict how it would have played out. Would the Kerensky Offensive have done better if the Germans had sent more troops attack the French? Would the French troops have rallied if confronted with an actual German offensive?
Regarding 1918, the Germans were pushing so hard and getting stretched because the AEF was streaming in [1]. (American divisions were involved in plugging holes at the Second Marne BTW.) The Germans had a superiority of 300,000 men going into spring 1918, and inflicted 150,000 more casualties than they took during the Spring Offensive. Unlike 1914, there was no rush to beat the French before the Russians mobilized. If it wasn't for the US intervention, it would have been offensive after offensive until the Entente caved in.
Even without US involvement, France, Britain, and Germany were effectively all devoid of manpower. Once the Spring Offensive failed, there really wasn't any ability on any side to carry on offensive operations, at least to the point of producing a clear end to the war. So the first country to fall apart from the stalemate would be the one to lose it all. And Germany certainly seems the strongest contender for being the first to fall. It was having problems keeping even its soldiers well-fed, and the Spring Offensive produced a very indefensible salient. Germany's hopes of victory died at the Second Battle of the Marne, and they probably would have died there even had the Americans not started streaming to the front lines.