If I remember correctly, there was an inquest in the UK a couple of years back into the death of a stowaway on a BA flight.
I believe there was evidence that cause of death wasn’t hypothermia or hypoxia per se; what did for him was the rate of change of altitude, which was more than the body could cope with. In other words, the stowaway probably died before TOC. It may well have been the same this time.
Try a search, as it was definitely discussed here – I think Flying Laywer might have been involved in the case.
There are examples of people surviving, but they are very rare exceptions. Not a nice way to go, and you can only wonder at the lengths some people will go to for a ‘better life.’