Because it is "obvious" that a big enough bomb can bring down a large building does not mean that there was an easy fix at the time of construction. Evacuation routes would be safer is there were stairwells in all four corners (assuming the building is rectangluar). However, that would eliminate the corner offices with windows looking out in two directions and that command the premium rent. It is possible to construct a large building that would be relatively impenetrable to large aircraft by adding a hard shell to the reinforced core columns. Besides adding cost, that would also eliminate windows. The chief attraction of tall office buildings is the ability to look out over the world from the offices. A 100 story building with no windows would be largely unrentable. You might as well build underground. I am guessing that for at least the next generation, new construction in major cities will be buildings about the same height as surrounding buildings so nothing sticks out.

rich r