With every occurrence of a black swan event, you are made more resilient in preventing an event of a similar kind. So, now that we know that 9/11 can happen, we're now more mentally prepared than ever to prevent something like it if it happens again. Case in point, pilots now carry weapons and are much more likely to thwart attempts of hijacking. Passengers will also react more violently and vigorously now towards any sign of threat. Note that we actually have a few precedents since 9/11 in which hijacking of airplanes were thwarted in this manner.