Trabal knowledge gets forgotten too, just over longer timespans. One day, planes will fly with no one on board having been alive when 9/11 happened. They may not make the same assumptions about intent as adults today do. "Ever" again is a little long to be so sure.
"Tribal knowledge" of airplane hijackings was why 9/11 worked initially.
People sat still because a "standard" airplane hijacking meant going to some odd airport while a bunch of "freedom fighters" got their 15 minutes of fame and then everybody on the plane got safely released a week or two later.
Once people realized that the 9/11 hijackings instead were suicide runs, the "tribal knowledge" broke and people didn't sit still anymore (see: the fourth plane on 9/11 or the shoe bomber).
