I do agree that ISIS is going to be a short lived phenomena.
By definition assuming the analysis of them non-rational players with an Islamist apocalyptic mindset is correct, then they'll run into the ultimate barrier that all apocalyptic cults have when the apocalypse doesn't cooperate. Western and various apostate Islamic factions show up to fight them, and ISIS doesn't win, Mohammed doesn't show up, and the 12 Imam doesn't jump out of the well or whatever... They'll lose steam.
Then the PITA is going to be the nastiness of a wide array of splinter groups metastasizing and re-forming, but it won't have the short-lived unity ISIS is demonstrating.