> at a minimum it prevents wildly unstable people from boarding with weapons
The TSA reliably fails penetration tests; they miss between 70% and 95% of them. https://www.forbes.com/sites/michaelgoldstein/2017/11/09/tsa...
What's likely caused hijackings to fall off a cliff is a) reinforced cockpit doors and b) the knowledge of crew and passengers that the hijackers might be homicidal.
Pre-9/11, advice was to cooperate, get the plane on the ground, and negotiate/assault, with pretty good overall results for the passengers. That is... not the advice today.