The interesting part to me is that the mode of attack for 9/11 can't easily be repeated. It depended on passengers assuming the plane was being hijacked, and that they would eventually be released. That stopped working on the 3rd (edit: 4th) plane. I don't see how it would work again. Passengers now default to assuming there's no path to live other than fighting back. Meaning many of the measures we still take aren't…
Any attempt at this, the terrorists will have a game plan of how to convince people on the flight to chill out, that they have no mal-intent towards the passengers, etc etc. I mean it might not work, but I reckon they'd put on a really good act and manipulate people heavily into making it feel like they are actually quite safe and it would be a poor decision to try and overrun them.
If we want to prevent hijacking of planes for terrorism purpose, then we'll need to go deeper and fix the root cause of extremism which required tons of focused effort, probably tackling seemly intractable problems. Not spend billions upon billions more on likely ineffectual screening procedure and tools to satisfy the surveillance state.