So why did the missing 777 go to 40,000 feet??? Because the Pilot Oxygen Mask is rated to 40,000. The passenger masks only to 25,000 feet.
This is the system supplier to the Boeing 777...
http://www.avoxsys.com
http://www.avoxsys.com/pdf/products/crew-masks/359-series-crew-oxygen-masks.pdf
So we go to 40K feet. Depressurize. Masks Drop. Everybody dies anyway. Now back down to 25,000 and off for 8 hours of quiet flying to.... somewhere.
Plus the o2 generators only last for a specific time, its not like a system of oxygen tanks throughout the plane.
Last I heard that time frame was 8 minutes.
Pilots get pressurized o2, at over 25,000 feet from what little I know you need it at a higher pressure as your lungs just cannot expand enough, but what do I know, I’m sure an expert knows more.