The cost of such a structure would be enormous.

Skycrapers are an ego thing. If they aren't pretty, nobody will put up money for it. The WTC comprised of 8 city blocks. With 8 city blocks to work with, one could build a plain vanilla building or buildings that aren't all that tall and have a lot more office space. The reason for building huge skyscrapers is that they are cool. Take away the coolness, and you take away the need for a skyscraper.

The way skscrapers are designed will probably get a lot closer scrutiny now. Not that this wasn't the case already, but nobody designed buildings to withstand airliner impacts.

Aircraft have hit skyscrapers before, but never to my knowledge has it been a deliberate attack before. on July
28, 1945 a B-25 Army Bomber enroute from Bedford, MA to Newark, NJ crashed into the 79th floor of the Empire Sate Building. 11 people in the building died as well as the pilot and three members of his crew. Only 26 others were injured. It was a Saturday when the pilot, blinded by fog hit the building. Ironically because of the fog, not many tourists were in the building.

Will we quit building skyscrapers? I doubt it. Buildings are built to withstand collapse for 1 hour, so people have a chance to get out and be rescued. We may see large buildings such as the WTC be required to withstand collapse for 2 or 3 hours. This may result in asbestos being used again, or they can start encasing the steel in concrete. The main disadvantages to concrete is the weight. You then have to beef up the steel to carry the weight of the extra concrete. Concrete is very hard, but is not tensile. When the building sways in the wind concrete will want to crack. Expansion and control joints will have to be carefully calibrated to keep the concrete from cracking up over time. Asbestos is only a problem when it's airborne. Obviously there is a big asbestos scare going on, but we may see a comeback unless they find a better fireproofing material.

I am sure some very smart engineers are already rethinking the way skyscrapers are built.

-Steve G. (also not an engineer, so I welcome any engineers to fix my mistakes)