You have to breathe...so you either have the O2 system to stay up there at alt, providing for you and pax, or you come down to an alt where you the O2 system can provide for you and pax, or you come down to an alt where you can now breathe ambient air...What ever alt you end up at, to breathe and not be a pilot Popsicle, now you have to consider your new fuel burn, that if planned for, you can make to your destination.
Can your average airliner leaving LAX, once hitting the half way point, continue at alt providing O2 for everyone or do they have to come down and now have enough fuel at the lower ALT to make it to Hawaii.
It's a loaded question and I would really love someone to show me how 150 passengers could be provided 2.5 hours worth of O2 at 39000 feet.