Didn't many people attribute the collapse of USSR to President Regan's 'StarWars'space defense program that was too expensive for the USSR's military budget to keep up with? That US either in reality or by fear coerced the USSR into the belief that they would nolonger be able to launch a successful missile attack against the US because we would have SDI? That if they spent as much as it would take to defeat or defend against SDI they could not spend on their social programs to the extent that the Soviet people had come to depend on them?
So given the intense competition the US has with China for energy and other economic doings. And given the commitment US has to it's citizens in various social programs. The terrorists just have to make us spend so much money trying to defend against what is really almost indefensible, a surprise random attack. The terrorists can just force US to spend more than it can economically handle and collapse the economy into a massive depression?