They will not allow them to exist in Syria if they want their puppet in control.
BugI, I pretty solidly agree with all of this, but still feel that "announcing" the pull-out on a call with Erdogan was a huge mistake and horrible optics.
No, I don't think that is the case. It isn't the binary decision you present. I think (could be wrong) that baked into that analysis is the Powell Doctrine, "you break it, you own it", which in my opinion is ludicrous. if it is in America's interest for us to be engaged, then engage. The minute it is no longer in our interest, leave.
I do believe that the Kurds with a homeland would be in our long-term interest, but don't believe that we should be in the business of "creating" a Kurdistan. I could be very wrong on this and there might be folks far smarter than I who might see this as creating another India/Pakistan eternal feud.
IMO, Russia's game plan for ISIS was solely to use them and the anvil to the Russian/Syrian hammer on the anti-Assad rebels. Perhaps there was more, but I didn't see anything that led me to believe that taking on and eradicating ISIS was ever in their game plan.
Eradicating ISIS never seemed to be our goal either. In fact, I'm not really sure what the goal really was.