You won't be able to determine what comes after the defeat of Islamic State by ignoring why it gained so much territory in such a short time. In Iraq, IS grew quickly because it had genuine support within the population of north-east Iraq. In Syria, IS made great territorial gains because the Syrian government was locked in conflict with rebel and terrorist groups in the west of the country. IS in both areas further benefited from the support of Saudi Arabia and its Gulf allies, with the USA's tacit approval.

The USA allowed Islamic State to arise in Iraq because it pleased their Gulf allies, and because defeating it would mean fighting Iraqis on their home turf. The USA allowed IS to take over huge portions of Syria because that furthered the goal of destroying the Syrian government. This again pleased their Gulf allies, and Israel as well.

Islamic State has been an extremely useful tool for the US, and in one form or another will continue to be so.