The Kurds saying ........No Friends but the mountains .......is a true saying.........
There are 3.6 million Syrian refugees in Turkey:
Situation Syria Regional Refugee Response
Other refugees have crossed into Europe, around 500.000 alone in Germany.
They all need to return back. And that will be the outcome of the political solution in Syria.
Backed by US airpower PKK/SDF/YPG has expanded in sparsely populated territories (refugees) in Eastern Syria. USA made the remaining Arabs work together with Kurds in so-called SDF alliance, probably even paying paychecks for the fighters or letting Gulf countries pay for it.
With USA gone PKK/SDF/YPG influence will be rolled back and the Arab charachter of Eastern Syria restored. USA is not willing to engage in nation-building or reconstruction of areas like Raqqa.
Plus, there are frictions with Damascus, Russia, Turkey.
Without US Airforce Kurds cannot hold territory or play regular Army.
You said it yourself, that mountains are best friend of Kurds.
Problem is there are no mountains in East-Syria.