Most of those who died in the WTC's on Sep 11th 2001 did so because of poor structural design of the building, the planes were merely a catalyst. If the buildings had been properly built the death toll would have been much lower.
I believe it's unreasonable to expect architects to take into consideration the fact that their buildings might someday be crash targets for 2 or more full size airliner jets. In my opinion, the WTC took quite an unexpected beating and handled it remarkably well. Can you even begin to imagine the tremendous forces involved when the first 767 crashed into the WTC? When was the last time you surveyed the entire width and mammoth size of a 767?
Let's not lose perspectives here; most of those who died in the WTC on Sept 11, 2001 did so because of a ruthless terrorist attack, the sheer grandeur of which was before now utterly inconceivable, and not because of any architectual design flaws.