I think 50,000 ft. might be a typo, I’m sure you meant 40,000 ft. The C-5 has been tested up to 43,000 ft or so but AF rules only allow 42,000 feet max for transport aircraft (There are a few exceptions) the constraint is the O2 masks, the ones installed on transports and airliners don’t work so well above 42,000 ft when depressurized. Also the self contained passenger oxygen systems only last 22 minutes. It takes a transport aircraft about 22 minutes to descend to 10,000 feet without over speeding or over stressing the air frame. I’ve been at 42,000 feet in C-141s and KC-10s trying to get passed thunderstorms. Do not like! I’m more comfortable at or below 39,000. Most of the time we can’t reach those altitudes due to weight and performance limits. Usually only after you’ve burned off a lot of fuel, but by then you are so close to your destination that you don’t want to climb higher just to descend shortly after. Edit to Add: another constraint is your time of useful consciousness is at those altitudes is 10-15 seconds if you experience a rapid decompression. (Less at higher altitudes.) That’s barely enough time to recognize the situation and get your mask on. One pilot needs to be on oxygen when flying that high. Most aircraft that fly above 42,000 feet require all pilots to be on oxygen, although I’m not sure about the Concorde or Gulfstreams etc.