Given that the impacts and fires were not survivable, it was probably better that the Towers slumped inwards and down rather than toppled.
If the Towers had toppled through failure of their outer walls before their cores, the damage to neighbouring structures would have been enormous. Loss of life would have shifted from the occupants of WTC to occupants of other buildings.
Informed letter from Mark Whitby, Civil Engineer to The Times, today (27 Sep 2001) sets this out clearly. Whether this was luck or was planned in design we have yet to be told. But he implies that this was the least worst outcome. (I can't use the word "better" again.)