I agree with you, that a sudden US withdrawal would simply throw the place to the wolves, and our Kurdish allies would take huge casualties. Iran and/or Erdogan would be the likely winners.
One of the great things about stabilizing Kurdish/SDF rule over Eastern Syria, is that the Euphrates River Valley between Raqqa and Fallujah is the heartland of the tribes that formed the backbone of ISIS - and the Kurds are notoriously excellent at internal security. I can’t think of any group that could more effectively police those populations, even Israelis.
Keeping the Kurds/SDF in charge there will steadily reduce the ability of ISIS, or some offshoot with a new name, from re-forming in those areas - and give us superb human intelligence of what is going on across the river in Assad/Iranian territory, or down the river in neighboring Iraq.
Iran has shown that they will maintain and operate Sunni terrorists or jihadis when they can manage it, in their long term hosting of high level al Queda Shura Council members in Iran - including bin Laden’s son and heir apparent (who was later killed trying to visit in Pakistan). Iranian-supported Shi’te militias are a blunt tool at the whole Sunni community and vehemently anti-American, whereas the Kurds wield a deft scalpel and are strongly pro-American.
The Kurdish Regional Government in Iraq, and the Syrian Kurdish police/security (Asayish) have both shown an uncanny ability to monitor and control jihadi violence in their areas of responsibility.
Having good internal security will be very important for the US Forces in Syria, as Iranian LTG Suleimani has already threatened that he will make American presence unsustainable (by picking off troops, like he did in Iraq). When American soldiers were dying in Iraq by Iranian-made explosively formed penetrators (armor piercing shaped charges), none of those deaths occurred in the Kurdish controlled areas. None. They provide about the best security that you could hope for.
Other than Israel, our best allies in the region are definitely the Kurds. It’s a shame but the Kurd diaspora in Syria, Iraq, Turkey and Iran truly deserve their own country, but I just can’t see it realistically happening as that would just start another hot war and who knows where it would go.