In fairness to the WTC buildings - they actually performed very well (although that might seem difficult to appreciate under the calamitous circumstances involved) they maintained sufficient structural integrity to allow escape during the period of over an hour after each of the initial collisions which is quite impressive given the temperatures of 1000+ deg.
The real issue though would seem to be whether high buildings are 'escapable' in the event of a calamity - certainly the experience in the far east has not been encouraging.
As to the argument that low level buildings are equally vulnerable - this is quite simply incorrect - for example the pentagon buildings which accommodated a similar number of occupants suffered a reasonably low level of casualties by comparison to the WTC (125 V 5500 casualties).
High buildings cannot be made safe or escapable in the same way as low rise buildings. The WTC buildings performed far better than was to be expected however probably far better than contemporary structures would do under the same circumstances. Unfortunately, it still remains that tall buldings are pretty inescapable in such circumstances - at best fire can be contained and structural collapse minimised.
At 'breakneck' speed it takes a single individual 10 - 15 minutes to clear 35 floors through an open and clear stairwell - imagine that same stairwell filled with other occupants, shrouded in smoke, blocked by debris and that 10 - 15 minutes could turn into 30 - 60 minutes which is usually the outer limit of integrity in the case of fire.
So no answers then to the escape dilemma - Can structures be improved - hardly, in most cases even the most up to date technology finds it difficult enough to deal with 'normal' fires. When circumstances lead to a fire of 1000 deg it is almost impossible to guarantee performance. As to the specifics of such damage - it would not necessarliy take an airliner laden with fuel to generate such temperatures - a gas explosion or fireball within an air conditioning duct can generate similar temperatures over a very short period of time which none the less will cause serious damage.