The world will be better for the total destruction of Daesh/ISIS etc. They are essentially a death cult with many similarities to the Khmer Rouge. Implicit in their vision of return to the seventh century is that there are too many people now and they are too diverse. Their claim to Islam is effectively exploded by the Kurdish opposition, who share their Sunni traditions, but are not Arab and fight for emancipation and modernity.
It was the right tactical move for the USA to involve these superb fighters in containing Daesh to the north and west of Mosul, and to put them on the front line in Raqqa. It’s not the first time this battlefield alliance has occurred. The CIA encouraged them to battle Hussein in the 60's, and then abandoned them for “strategic” reasons. The vile war criminal, Kissinger, remarked “Can’t they take a joke?” as Hussein murdered them. Similarly Bush senior left them supported after Desert Storm, stopped short of dislodging the dictator.
It would take a considerable strength and strategic vision for the US not to repeat this pattern in the face of Turkish opposition. If northern Syria remains even a semi autonomous region, then Erdogan will have difficulty, taking the 25% Kurdish section of his population, on his journey to a theocratic police state. The Kurds deserve our continued support and are the only hope of establishing multi party democracy in the region.