Second, the issue is the marginal expenditure to marginal deaths. We spend tremendous effort and some freedom reducing auto accidents, and we reduce them substantially. We don't spend trillions more at this point, because we don't consider it worthwhile. Before 9/11, we had some level of expenditure (of effort, money, freedom) on preventing terrorism, and doubtless saw some reduction in terrorism over what it would have otherwise been, and had it down to some particular level. Since 9/11, we've been spending that plus trillions of dollars and more of our freedoms, and the most we can possibly do is reduce terrorism to zero - and even if the alternative were a significant increase in terrorism (something like 9/11 every other day, instead of - generously - once every several years) there would still be more room to save lives in these other ways.
Finally, 9/11 had an enormous cost, but a lot of the psychological cost is due to fear mongering from people who lack the perspective the numbers bring. Another large portion of that cost is due to lack of resiliency in the companies with offices in the WTC, and companies doing business with those companies, &c. The former is addressed through education and leadership, the latter by encouraging behaviors that will also help in the case of (say) a massive hurricane. None of this is served by a panicked focus on terrorism.
