This gets into the broader picture of Iraq, where I think the decline of ISIS is not the decline of Iraqi jihadism. Even if ISIS were to cease to exist as an organization, it would be replaced by another organization that I think would be less brutal, but you would still have problems.
The forces that would be there to deal with insurgencies would obviously be the peshmerga in the Kurdish north, the Iraqi security forces, and you still have Iranian-backed militias which would be a part of the response. Iran has a large number of commanders on the ground, including those in Quds Force. A number of Quds Force commanders have been killed in Iraq. So you have a number of different forces on the ground. Obviously, there are also American and Canadian and British forces.
I was in Ottawa when news first broke that Canadians had actually been in engagement against ISIS and not just playing a supporting role. I know what big news that was. So you have this array of forces that would deal with it. But if they are forced to resort to just being an insurgency overall, then as I said, that means they have more difficulty holding territory, which in itself is a sign of ISIS' decline. Now, this is probably going to happen in Iraq, but not in Syria, because there's not really a strategy to displace them from their holdings in eastern Syria, which means they'll be a regional problem for some time to come.