Look at the map, e.g. the following one
https://www.aljazeera.com/indepth/in...144229467.html
Even this anti-syrian source admits that the major part is controlled by the government. Putting the kurds aside, the question arises: where are so-called "coaltion supported" "democractic" forces? ..... "Vanished in the haze".....
A small area around Al-Tanf is just a safety zone around the US base (to be shut down according to Trump). The Idlib zone (where re-named Al-Qaeda and An-Nusra terrorists reside) was agreed to be under the turkish resposibility (to avoid gov. forces offensive). And Turkey now has a difficult task to demilitarize it.
So, after the kurds were "sold out" and talks between the Syrian government and "moderate" opposition are on the way (a so-called "Astana process", out of any US control), pulling out of the area seems a logical solution.