It is nonsense to assume the stalemate would have continued indefinitely. Both sides were exhausted. If we hadn’t intruded a peace would have had to been made.
Oh, I agree.
Here's the problem: neither side was able to strike a killing blow, and neither side was willing to compromise enough on their war aims to come to real terms.
If the US had not intervened, the Germans would have retired comfortably to their lines beyond the Marne for the winter and, come spring, either the Allies or the Germans would have tried again.
It could have gone on for two or three more years.