It will destroy ISIS. The means of its destruction won’t be pretty, but it will happen. If we had allowed the Northern Alliance to finish off the Taliban, we would have saved thousands of American lives. They would have massacred hundreds of thousands of Pashtuns, and driven millions into Pakistan, but that would have solved the Taliban problem. No Pashtuns in Afghanistan, no Afghan Taliban.
It will likely drive Isis out of the desert and into the cities where it will find refuge among Sunni militants. Yes, it will degrade Isis but it probably will not destroy Isis because there will be no boots on the ground ferreting out these fanatics from every hole and corner in Iraq's cities. Later, Isis will emerge and resume its quest for a caliphate.
The means of destruction can’t be any more ugly than what ISIS has broadcast to the world. So long as they keep publicizing American deaths, our will should remain strong and fairly united. We couldn’t let the Northern Alliance finish off the Taliban because their effective leader Massoud was assassinated at the critical time. Like driving anyone into Pakistan is keeping them from coming back, yikes.