I don't think that it is that unlikely, provided that the procedures are practiced. Flying at great altitude is not new. The military were regualrly doing it in the 50s and the 60s. The Venom could get up to fifty plus and the Vulcan often flew well above fifty thousand. The crews were required to train to cope with an explosive decompression by pressure breathing and were required to regularly refresh. It is true that the time of useful consciouness is small without pressure breathing and it's also true that military crews, when they flew at that altitude, kept their masks on, even though the cabin was pressurised.
I think that it's possible provided that you practice finding and fitting the mask in a hurry.