Yes, actually, it WAS luck... we've had failures during launch, failures while in space, and now we have a failure during re-entry... just because something never went wrong before doesn't mean it never will...
Secondly, I'd like to state that we have NOT had 42 years of successful re-entry with space shuttles - in fact, this particular re-entry was with the heaviest load ever - never before have we attempted re-entry with a load as heavy as was attempted the other day...
"Facts" may be on your side... "Reality" is on mine...
So I suppose this load was never calculated as to its affect on re-entry. It was just some miscalculation or non-calculation. Please, this would be worse than failing to protect the tiles on liftoff.