They simply die with us if there's no other person that has access to them. In most cases it's pretty obvious that people don't have access to whatever crypto currency we possess because of the safety of these assets, trying to prevent them from being stolen so we choose not to share the access with anyone even to the point of family members. So If no one knows about it then it becomes lost the same way we lose these cryptocurrencies whenever we lose our recovery phrases or any other means of getting them again