Turkey will create the buffer zone anyway. If the US military stays there, then they get surrounded bases that are good for nothing (and it would lead to expensive logistics). The Kurds can fight or leave this zone, like they did in Afrin. The rest of the Kurdish territory is safe, Turkey will not invade the whole east of Syria. The Kurds will still have the oil fields as bargain chips, that's why they haven't called Assad for help against Turkey.