Middle East scholar Daniel Pipes thinks ISIS will eventually implode:

As in every other aspect of life, the Islamic State unabashedly and brutally applies premodern Islamic law, making no concessions whatsoever to modern mores. It seeks to establish a universal caliphate as though it were again the seventh century. Beheadings and enslavement being among the most shocking Koranic injunctions to a modern sensibility, the group most exults in precisely these and imposes them on those it considers infidels.

The wild, reactionary impulses of the Islamic State appeal to a minuscule number of observers, while its messianic zeal has carried it very far, very fast - from the borders of Turkey to the outskirts of Baghdad. But its actions appall the overwhelming majority, Muslim and not, which will lead to its inevitable collapse while it does irreparable damage to Islam.

Let's hope he is correct.