There WILL be another 9/11. This stuff happens, and it CAN NOT be 100% avoided.
You can do some things that could help (hint: the issue with 9/11 wasn't a lack of info, but a lack of communication. We still have this problem regardless what the NSA does), but you can't stop it from ever happening.
So, no matter what things you do to prevent 9/11, something like it will happen again. The Boston Marathon bombings, while much less severe, show that even with the super surveillance, people dedicated to cause death can do it, and always will be able to.
What makes for more interesting discussion is if there are ways of preventing the root causes for wanting to cause death. Perhaps that can be more effective than ignoring what the cause is & trying to stop the effect?