ISIS grew thanks to Assad. Assad was deliberately bombing towns of opposition and sparing towns of ISIS. You can see devastating destruction in Daraya, Jobar, Homs, Aleppo, Douma, Zabadani meanwhile Raqqa, Al Bab, Manbij have controlled by ISIS have very little damage. Assad also gave to ISIS many bases full of weapons (17th division, 93brigade, 121 brigade, Tabka airbase) virtually without fight. In same time Russia
encouraged thousands of Chechens to join ISIS.
Thats how ISIS grew in Syria. Once Assad is gone there will be no reason for people to stay in ISIS.