Also logic is missing here. That Trump ends the support for "rebels" doesn´t increase Syria´s need for Russian air support.
Believe me, it is a good decision that Trump made. While the US could maintain one or two small bases in southern Syria for a short while, this would be without any strategic value because neither can the "rebels" capture ISIS held soil nor can they interrupt Syria´s newly established connection with Iraq but it would come at then cost of better relations with Russia. They are totally encircled, outnumbered and outgunned by the pro-gov forces and all they hold is a territory that consists almost exclusively of desert.
The other "rebels" are out of control of Washington and at war with the US-backed SDF.
And of course, they are terrorists need to be rendered harmless.
"Moderate" jihadists in southern Syria with US troops (note the ISIS finger):