Hudson,
Your assumption that oxygen was a reason for not bailing out above 10,000ft. is partially correct. However, the best reason for waiting until reaching lower altitudes is for higher temperatures. If you took the example of going over the side at 35,000ft, the standard temperature at this altitude is -34degs. By the time you reached the earth you would be a "frozen Hudson", forget the oxygen concern. As the altitudes decrease, the temperatures increase and so do your chances of survival. You can ascertain the temperature at any altitude (approximately) with the formula for the dry adiabatic lapse rate which is 3.5F degrees per 1000ft or 2deg.C. per 1000ft. of altitude. Simply start with the surface temperature and work up. Then, the "wind chill" factor (falling at 135mph through cold air) must be taken into account. On the ejection seats I am now using, the berostat is set to start the opening sequence at 10,000ft if the ejection is made at a higher level. But, we do have oxygen during the decent, something the Mustang and other types of that period did dot have. Having flown the Mustang, I remember that if you forgot to duck your head while pulling the red canopy release lever under the RH canopy rail, you could forget getting out anyway.