9/11 will never happen again because the idea of a plane hijacking for ransom no longer exists in passengers' minds. If someone hijacks a plane now, the passengers assume the hijackers have suicidal intent and will curb stomp the hijackers to death or die trying. So in that regard I think TSA confiscating scissors and pen knives is just silly. A simple metal detector is sufficient for finding large weapons like guns.
On the other hand, terrorists' only option now is to just destroy the plane and kill all the passengers onboard. Cabin doors are locked, so this involves blowing up a bomb concealed somewhere (checked luggage, shoes, liquid explosives, etc). How do you propose we detect whether someone is trying to smuggle a bomb on a plane without something like the TSA? Just keep an eye out for nervous passengers and report them?
In other words, if we abolish the TSA tomorrow and replace it with metal detectors, what's to stop someone from bringing a few gallons of chemical explosives in their backpack, mixing them in the lavatory, and blowing up the plane? TSA has always caught when I accidentally left a full water bottle in my backpack.