As bad as the 9/11 attacks were, it was a single incident that paralyzed us for a few days, and impacted us for a year or two, financially speaking.
My concern would be a series of attacks that called into question the viability of big cities, and the ability of the .gov to protect us. If we have few follow-up attacks, who then would venture forth back into the city for fear of the next attack? If they Bad Guys can cause an actual economic collapse* here, then our ability to act on the world stage, to defend Israel, prop up "moderate" governments in the ME and south Asia would disappear. That is what the Bad Guys REALLY want, and it seems all too possible.
*By economic collapse, I mean something much worse than what we're seeing now. I mean a breakdown in the division of labor. People making their own clothes, growing their own food, etc. That's what concerns me.