Should the passenger emergency air be able to maintain life in a decompression at the maximum altitude achievable by the aircraft?
That would make the safety demonstration very interesting - by the time you have shown 300+ pax how to don pressurised masks and then how to breathe through them, you'd be at your destination.
And just how long would you plan on being able to sustain life for at FL400 with no cabin pressure? Where would you store all the extra oxygen that would require?