I choose to see this question as “If you could magically just make someone a billionaire, who deserves it,” or more specifically “who would actually do good things with the money if they had a billion dollars.”
As you said, the reason these people aren’t billionaires already is because they haven’t been exploiting others. That being said, there are likely a few people that would use the money to better support a lot of great causes, like the Free Software Foundation, medical research, or climate change action