I agree that planes ARE designed to withstand this kind of stress. This one may NOT have been built well enough. Also, there is a term called Vne, or velocity never exceed, for all aircraft in a variety of attitudes and conditions. The wing vortex of a fully loaded 747
MIGHT be able to cause very localized conditions that exceeded Vne for th A300. This would be especially true if the structure were weakened.
What's my point? That violent wake turbulence is plausible in this tragedy, BUT should not have knocked the A300 from the sky, and should NOT have knocked off the vertical stabiliser.