At the end of the day, 9/11 was a one-time event in my mind. It worked because we never imagined someone would weaponize an airliner full of people, much less 4 of them simultaneously. It worked because it hadn't been done before. Now we know that, and even the general passenger won't allow it to happen again. United 93 is proof of that. As soon as the passengers realized what was going to happen, they took action. The key now is to look ahead to the next event, and try to predict and prevent it.
As far as the screeners go, TSA or privatization I think is 6 in one hand, half dozen in another. It can be pro/conned to death from both angles. My one opinion is that public safety is easier done by government than private industry. Private industry is going to be profit-driven by its very nature. Doesn't take long before profit takes precedence over life safety. Look at some of the dirty things we've seen from big industry, all to save a dime. Granted government has its own problems, and isn't known for efficiency either.