ISIS itself is more concerned with seizing territory around their borders of influence, and expanding. Logistically, offshoot cells can attempt one-off terror attacks, but that threat is no different since 911 or before. Each day, no doubt many such plans get foiled in various stages.
If you really do the research, ISIS is not your average group of terrorists. They are trying to create a real government, etc. and are doing it using terror as a tool, but unlike a gang, they are not just looting and pillaging, but trying to establish a Caliphate Islamic state.
I think they are delusional (and that's what makes them really dangerous), as there is no way the international community is going to tolerate it, when keeping ISIS around no longer suits their goals. Even if they "won", nobody beyond their existing trade partners are going to trade with them, and indeed, they'd soon do something stupid again to get the world's ire on them, and then a redoubled effort to remove them.
Right now, ISIS' presence allows the US and Russia to sit on the Syrian sidelines and not lose face. When that prospect goes away, that's when you'll see a final, ground-based solution to ISIS, but not before.
