There is no reason to believe that an effective effort by outsiders to wipe out Isis in the territory it controls would do anything to reduce the likelihood of Isis related atrocities elsewhere. It could very well have the opposite effect.

That in itself might not be a reason not to act in that way. But it would be foolish to imagine that it would be a simple cost-free option.

Rushing into action more or less at random, in order to be seen to be doing something, in the way that happened in the wake of 911 would be likely to be as disastrous as that was, with all its consequences - which in truth include Isis.