It’s good to finally dispense with the phony idea of everyone dying at 45,000 feet ... LOL ...
With the cabin pressurization turned off, there's not enough oxygen available at FL450 to remain conscious for very long and probably not enough to live for much longer, depending on one's age and physical condition.
The oxygen concentration is about 21% at sea level and only 6.8% at 29,000'. I don't know the percentage at altitudes greater than that or what is required to live.
Regarding Payne Stewart's Lear, they probably froze to death in their sleep if the oxygen deprivation didn't kill them first.