When there's a will there's a way. They could make a building able to withstand 10 747's but that would only mean that they would use 11. Not rebuilding will not prevent someone from trying to fly an airplane into a building. We can't cave to fear, that's not what this country is about.
Reading some of these post in this thread reminded me of an anti-violence example that someone told me after 9/11.
1. Find someone who's against a violent solution against the groups responsible for 9/11.
2. Punch that person in the face.
3. After they get up and come after you remind them that they are against violent retaliations so hitting you would be against their believes.
4. After they realize that they wouldn't be right to strike you hit them in the face again.
5. After they get up remind them again that it's against their belief to hit you back.
6. Now explain to them that there are times when the only way to stop getting hit is to hit back.
Remember all those airliner highjackings in the 80's funded and accomplished by Libyans. Remember how they stopped over night when Reagan sent in those F-111's to send a message. Message recieved.