The expansion of Da’esh means the fragmentation of Da’esh. It will follow the same path of every extremist group; it will fracture and divide, and spawn new sects that will each claim to be purer and closer to the truth than the other, and they will all fight each other.
Da’esh as an idea will not end, because as an idea, it is not new; they belong to a long tradition of extremist interpretation, ignorance, and demagoguery. We have always had fringe radicals, and we always will. But as an organization, they will cannibalize themselves and die. This, in my view, is inevitable.
Anything useful that they have contributed (and they have contributed some useful things) will not ultimately outweigh the negative impact they have had, though we have not yet seen the full ramifications of this. Their main accomplishment will likely be the de-legitimization of jihad in the minds of the majority of the Muslims; and they will cause a major backlash against religious discipline and the call for Shari’ah, and very possibly usher in a new wave of popularity for secularism.
The majority of the youth who have joined them, if they are not martyred in Syria, will be stranded there. Once a political settlement is reached, and Syria is partitioned, whatever puppet regime is set up to manage the territory they are in, will promptly arrest them, and they will either remain in prison, or face execution..May Allah reward them for their intentions, and may Allah help them when they eventually realize that their intentions have been grossly manipulated and exploited.