Well, if the western coalition would not encourage Saudis and Qatar to support "rebels" who are Al Qaeda associates and where Daesh gets the weapons from, the Syrian territory would be already cleaned up from the terrorists.
And this strange policy when the "rebels" are nearly nailed down: the West starts pressure not to kill them, let them go out, arrange for multi-day fire-stops, etc. All this allows for the terrorists to re-group, get supplies and attack in another place, like it happened to Palmyra recently. Another threat may arise from Mosul: the fears are that the coalition would simply allow the terrorist go to Syria to make it more difficult to Syrians.