As I said, the French manpower situation was worse than Germany's. But the risk of socio-political collapse in Germany was greater because of the effects of the blockade - they simply couldn't deal with another winter. And that's not even getting at the horrible state of the Austro-Hungarians. And the US contribution of bodies to the attrition game was paltry by WWI standards - a little over one hundred thousand dead. Would an extra fifty thousand or hundred thousand in losses finally pushed France over the edge? Maybe, but it would have had to have happened quickly to save Germany. And note that the unrest you are talking about took place in the spring of 1917. In other words the French persisted for a year before the US troops started hitting the front.