Crushing ISIS reaaly doesn't make much of a difference in the long term. The underlying issues that enabled their rise will be as prevalent as ever (probably worse). They will splinter into smaller militant factions, until eventually the cycle repeats itself and they are once again consolidated into a centralized leadership.
Except the rise of ISIS is also linked to the way the US and Iraq handled the post-Saddam era. ISIS sprung up because Sunni-Baathists who long enjoyed power were suddenly stripped of all representation, removed from all positions of power. A lot of early ISIS fighters were remnants of the Saddam regime. They won't be replaced as easily especially as Assad's regime once it regains full control will kill or imprison everyone who participated even in the slightest. ISIS at times was also a symbol rather than an actual group. They tried to take credit for basically every Islamic terror attack. In places like Libya they filled a vaccuum left by Gadhafi's killing and removal.
Will be hard for any major group to gain power like this going forward for a little while. I don't see any major countries being toppled, with SA crushing the Yemen revolt, Assad and his forces with Russian support taking back large swathes of Syria. Iraq and the Kurds taking every last ISIS stronghold, seems doubtful.
Of course terror will resume under a different name, but you won't see this coordination for a while. ISIS basically looted US arms that went to Iraq, captured rich oil fields, etc.