There are currently around 3000 billionaires out of 8 billion people, about 1 in 2 million. Assuming billionaires have an average number of children, the propor

There are currently around 3000 billionaires out of 8 billion people, about 1 in 2 million. Assuming billionaires have an average number of children, the proportion of population who is descended from them must double every generation, regardless of overall birth rates. The factor of 2 million is about 2^21, so we need 21 generations. A generation is about 25 years. So our final answer is about 525 years from now, i.e. around 2550. This assumes that age of death retains the same bounds as currently. If old age is cured before 2550 then someone not descended from a 2023 billionaire might live until the end of the universe.