The attacks of September 11th 2001 are fundamentally not reproducible irrespective of whether there is _any_ security screening at airports.
The default assumption before that morning was that a hijacked plane would fly around for a bit, then land. The default assumption afterwards is that it will be crashed if a hijacker is allowed to gain control, so the calculus on passenger intervention is quite different.
How so? The delay between the hijacking and the crashes in the buildings for both planes were around 40 minutes... even if there were jet fighters ready to go at the time, the lack of knowledge of the hijacking being in progress for much of this time and the short delay make this kind of attack still feasible.
What was actually improved our chances to avoid such attacks are the limited access to the cockpit and processes pilots must follow in case of hijacking.
The measures at the airport are to limit the risks of hijackings to begin with.
Passengers have intervened in several other terrorists attacks and now regularly intervene for other (non-terrorist) threat passengers.
It is extremely easy to get weapons into the boarding area, people do it accidentally every day all over the country and the TSA's own testing show that their screening misses the majority. Doors and procedures absolutely help as does the passenger response. Airport screening, OTOH, is primarily security theater.