While the Kurds have vowed to defend the territory they currently control, it is hard to see them matching up against the well-equipped and powerful Turkish military. The only way out for the Kurds may be seeking protection from Russia and its ally, Syrian President Bashar al-Assad.
While Syria's Kurds have long wanted more autonomy from Damascus, they are not against the Assad government per se. In fact, Kurdish fighters fought alongside the Syrian military in some of the battles of the country's civil war.
Faced with a Turkish incursion, the Kurds might already be on the phone to Assad and the Russian military since they certainly would view Syrian government control as a lot better than a battle against the Turkish army.
Another big question is what will happen with the many displaced people in the former ISIS-controlled areas and the ISIS fighters now being held by the SDF. Former US envoy McGurk believes Turkey is ill-equipped to deal with the situation, while President Trump has made it clear that he does not want to deal with it any longer. It's very difficult to predict what will happen next in northern Syria, but the immediate future after a US pull-out certainly does not look bright.