Tacan, you missed one point. The money that you can put into rectifying something you think might happen is the potential cost of the accident multiplied by the PROBABILITY of the accident.
So its not "Lets spend $500 million to fix this" Lets say the probability (for example only) of the crew needing oxygen on a given flight is 0.001 (one tenth of one percent) and the probability of the oxygen not working is also 0.001. Your industry wide "fix" figure maximum cost is now $500. Maybe change the checklists or put up a placard.
You simply cannot guard against all stupidities, although I guess by the end of today some American lawyer will be sueing Boeing for"Reckless and carelessly designing the aircraft"