I once had a problem after giving blood, where I blacked-out. I didn't become unconscious, I felt "out of it", as if I was watching what was happening without being there. Then my vision went grey, and a nurse noticed I wasn't looking well, and asked "Are you OK?" - I didn't have the brainpower to answer. Then my vision went black, closing in from the outside, and she took over and she and a colleague physically led me to a bed to lie down with my feet raised, at which point I recovered my vision, and I stayed in that position for a quarter of an hour until I felt OK again. Looking back, the surprising aspects were that I remained consious but couldn't see - previously I'd thought that blacking out meant losing consciousness - and that I didn't realise there was a problem until it was too late for me to take any action of my own. Obviously the root cause of this was low blood pressure, but I'm pretty sure hypoxia from other causes would present similarly. Seeing the data on how rapidly the crew would have become incapaciated by a sudden decompression at their initial altitude, I think they acted exactly right. They had lost the safety feature that would have saved them if a fire or decompression happened, and they wouldn't have had time to get out a MAYDAY, let alone get the aircraft down to a breathable level, if it had suddenly gone from a possibility to an actual emergency. Gambling that it wouldn't happen is just not on!