I did a few trips to the US around 9-11 and after it felt like I arrived to a dystopia where everyone was considered guilty unless proven innocent.
The irony is that 9-11 will never happen again, and not because of anything to do with TSA or airport security whatsoever.
Passengers and bystanders on a plane will NEVER allow something like this to happen again. They will fight like hell to physically stop a threat of this nature, whether or not they claim to have a bomb.
In fact, this change happened so unbelievably fast that when passengers on United Airlines Flight 93 - the one that crashed in Pennsylvania - found out via cell phone that planes were being crashed on purpose, they immediately stormed the cockpit in an attempt to take control of the plane. Those terrorists would have had the living shit beat out of them probably to the point of death if the passengers of Flight 93 had found out what was going on before they were allowed into the cockpit.
This has happened numerous times since, where passengers are extremely proactive and immediate restrain people making threats of that nature.
Further, the cockpit doors are now locked and heavily reinforced, and pilots now know to never let anyone take control of the plane under any circumstances, regardless of what is said or happening in the passenger area.
9-11 could never happen like that again even if TSA didn't exist and not one single thing was changed about airport security in response.