ISIS isn't trying to goad us into war. They're waging war on us, and doing so effectively. The kind of probabilistic analysis proposed by the OP takes terrorist attacks as if they occur randomly at some rate we have little control over, like car accidents. That is not the case when you have an enemy actively killing you and you have to make a decision: fight back, or signal that you won't fight back. If the latter, the number of "micromorts" is going to increase exponentially.