I agree that planes going into buildings is the major security concern, but we fixed it. We've reinforced the cockpit doors and we've taught passengers not to cooperate with hijackers (the latter we managed within hours of the first attack)
Plus panic buttons & guns in the cockpit, new procedures for aircraft leaving their scheduled route, and passenger's reaction to a hijacking (e.g. one "bad guy" with a switchblade is nothing against 300 people who fear NOT doing anything will endanger their lives).
Pilots have a panic button. It's called squawking code 7500. Alerts ATC that there is a hijacking situation (7700 is general emergency.) Guns in airplanes are a bad idea for, well, quite a few reasons.
...we've taught passengers not to cooperate with hijackers
Since I'm old, I remember the decades during which we were taught to cooperate completely with hijackers. Of course this was always bad advice, as was eventually made clear even to the idiots on TV. It seems possible that if no terrorism "expert" had ever appeared on TV, 9/11 could have gone very differently.
While all of this helps, it's hard to say that it fixed the issue. As far as I'm aware, for US based flights, there have been a few bombing attempts, but no hijacking attempt since 9/11. It's hard to say if this is a result of these security measures, the TSA security, or just the fact that hijackings aren't that common.
> we've taught passengers not to cooperate with hijackers
Have we? Maybe US passengers have been taught that better than elsewhere, but we don't really know due to the lack of hijackings to test that theory.
And internationally, of about a dozen hijackings post 9/11 only two I think have seen the hijacker get subdued by passengers - in one instance probably because there happened to be six police officers aboard; in the other instance it was a lone hijacker with a nail file...
Incidentally, despite 9/11, the odds of surviving a plane hijacking are still very high - the persons running the highest risk of getting killed in a hijacking are the hijackers. As it happens, 9/11 did not result in a string of copycat hijackers - most hijackers appears to still plan to survive the hijacking.
