That's a nice straw man you've built there. Let's try an alternate scenario: don't let people die because they cannot afford expensive treatments.

Now we have something else to aim for rather than policing people's bodily autonomy.

alright we want a society where it is possible for people to sell themselves into slavery which it is not possible to do today, but we will remove the most likely incentives (currently existing in our society) that would cause someone to do something so stupid?

Given your premise of a world where a billionaire wants to torture you to death, I'm puzzled as to why you'd think that the legality of a contract he's going to break anyway would matter at all.