The TSA has never stopped a terrorist from boarding a plane. Since the TSA's inception, terrorists have been thwarted by passengers (eg. the "underwear bomber" of Christmas '09), and do note that airport security let the guy get onto the plane in the first place.
But hey, you go right on cheering for the actual erosion of our hard-fought-for liberties for the sake of imaginary security. The TSA does not exist to protect us, it exists to provide the illusion of protection. It stems squarely from the "OMG do something even if it's wrong!" panicky mindset of those in power who have a pathological need to give the public appearance that they're doing something, useful or not, and not a one of 'em in the bunch willing to stand up and say "don't panic, let's stay calm and figure this out" because THAT sure as hell doesn't get anyone reelected.
Thanks a pantload to dumb, panicky sheep like you willing to spend your freedoms like currency to purchase safety, bleating to the government to save you, and dragging the rest of us along in your massive herd of stupidity.
9/11 won't happen again, but not due to any government action. It's because on that day, everything anyone thought they knew about hijackers changed. Before, it was always assumed they wanted hostages to use as a bargaining chip. Now everyone knows differently. The next time terrorists try to hijack a plane, it won't be the TSA that stops them, it'll be the passengers with vivid memories of what happened last time, and won't allow that shit to happen again. When you know your captors WILL kill you, you're much more incentivized to fight them to the death, because then at least you have a chance.
Jihadist terrorists won on 9/11 but they basically screwed themselves for similar endeavors in the future. They played their best ace, but like Daffy Duck, it's a great trick but they can only do it once.