> which are the ones that drove plane hijackings to near-zero

9/11 itself was. The general view before 9/11 was that a hijacking is an inconvenience: you get an unexpected detour via Cuba and have a nice story to tell at the next holiday party, but that's about it.

The second it became clear that the plane itself was the weapon and that there was a very real possibility you wouldn't walk away from it, hijacking became virtually impossible as every. single. passenger. would now be very much motivated to fight in order to prevent a certain death. United 93 already made this clear: they found out about what happened to the other planes, so they tried taking back the airplane. They didn't get it back, but the terrorist attack itself definitely failed.

Even without any kind of TSA future hijackings would almost certainly be closer to United 93, as American Airlines Flight 63 showed in practice.