You seriously think that 100% of the population under IS control would just emigrate if it blew up, or that they are all willing to fight in the IS army?
And the WW3 is more likely to happen if, you know, US bombs someone the Russian supports, and keep supporting someone Russian bombs rightfully? Thats the worse case scenario right here in terms of diplomacy.
Why would Assad want to keep those people in his country after he liberates those territories after great cost of life of his own people? It will mean endless wars for him for the next 50 years. They are disenfranchised Sunni's who hate his government, that's why they accept ISIS rule over his. No I don't think 100% of them will turn into T's, but probably enough to be a major problem.
Bombing them alone won't defeat them, whoever goes in to finish them off is going to have to deal with 100's of thousands of IED's and boobytraps, suicide bombers, street fighting, the entire area will be destroyed in the fighting, where will the civilians go and who will they blame? Assad and his supporters will consider them traitors, I would.