That equation was never in place prior to the attacks, there were official scenarios that suggested this tactic, but passengers and crew would have been correctly expecting another hijack with detour to Cuba or something
pfft, now you have hardened cockpit doors that’ll keep you safe if you manage to SE your way into the cockpit (which at least on long haul flights departing from europe hasn’t been super hard IME)
A terror group could just have one of their operatives blow a million dollars on first class flights to achieve some VVIP-status like BA executive premier, then it suddenly becomes really easy to ask for things like seeing the cockpit during takeoff.
Or you could just charter a few large private jets, there wouldn’t be any other passengers to fuck you over and you’d be in charge of the fligut route.
I guess they’d shoot you down faster these days, but to mitigate that you’d just need to depart from a city airport and make sure they let you in the cockpit early on.
I’m absolutely sure there could be a new (single-aircraft) 9/11, and it wouldn’t even be particularly difficult to execute by a group of people that isn’t already under heavy government surveillance.