Unless they make changed, long term will be tough for ISIS. They are a fire burning through the region- locusts devouring crops. They take over by destroying. What the don't destroy, they take and sell for money- their main source of financing. That and kidnapping for ransoms. (though they did get a small oilfield and a Syrian refinery and are trying to deal some blackmarket oil which could provide some large income). That leaves little behind to sustain- they require new towns to conquer just to be able to continue. And they aren't making lots of friends along the way- pissing off survivors in the towns and cities they mowed through which isn't conducive to expanding their movement. Other "radical" Islamic movements have not been willing to support or join them.