My airline is not in the business of flying empty airframes where we would have the option of operating above FL400. But in case of rapid decompression, which is a rare occurrence, it's not a case of sudden death. Look at the TUC chart:
Time of Useful Consciousness
FL 150 30 mins or more
FL 180 20 to 30 mins
FL 220 5 to 10 mins
FL 250 3 to 6 mins
FL 280 2.5 to 3 mins
FL 300 1 to 3 mins
FL 350 30 to 60 sec
FL 400 15 to 20 sec
FL 430 9 to 15 sec
FL 500 6 to 9 sec