Now think about the worst-case scenarios of nuclear or biological attacks on several US cities. The actual number of people killed or injured might well run into the tens of thousands but the vast majority of Americans would be unaffected outside the local areas. Losses of property would be limited in scope to small areas as well. Compared to the vast expanse and population of America, it would seem to be a small percentage of loss. Life would go on.
However, the terrorists know all too well what economic havoc would spiral out of this scenario. Lost businesses and workers would fail to support other businesses, who would not buy other products, etc. It would be years, if ever, before our economic lives got back to normal. Couple that with global boycotts and there you go. America can survive, in the larger sense, anything they can throw at us. But the US economy may not. And once the US economy is down and out, there goes most of our power in the world.
You have a point, Osama has suggested that, but the real hurt came from the lives lost, here, on our shores. IMO, he underestimated our resolve, overestimated the value of economic disruption.