Oh good grief. This is NOT Vietnam. Not even close.
ISIS the terror group cannot probably be "destroyed," as any nut job can call themselves "ISIS" and do something violent, bloody and stupid. It will eventually fade away, though, as other organizations instead draw away the supply of fanatical nut jobs.
But, ISIS the self-proclaimed "Caliphate?" Yeah, that's kaput. I'm sure some nut jobs will still try to run around stirring things up again. But massive failure to achieve what you claimed the first time around, while bringing misery to millions who are now really p'o'ed at you (and armed) tends to squash retries. In this respect, I think a claim of victory is quite real.
No, the next problem in that region won't be ISIS. It will be something else. What Assad does next will be part of it. But there are plenty of violent, bloody-minded, stupid people running around that part of the world ready to make everybody else miserable given half a chance, along with leaders quite willing to use them either as pawns or excuses for their own ends.
And that's as far as I'll go with that, because this is veering dangerously into political territory. Let's just acknowledge that a bunch of horrible people have met their well-deserved end. And be happy that their ideology is therefore also rendered even more questionable in the eyes of those otherwise inclined to believe such nonsense. Nothing says, "You know, maybe you don't have divine authority on your side" like getting your handed to you in spades.