I've seen these arguments gaining prominence on the last few years. I don't think they care about Mozarts or Einsteins though. If they did, they would use their money and push for policies that would improve the conditions people live in, so all the would-be Mozarts would have a chance to actually becoming one. I think they're worried global capitalism will collapse if population growth slows too much; which is probably true.

I also wonder if these people are funding all the anti-abortion and anti-contraceptive campaigns around the world... After a quick Google search, I see it's definitely been on Musk's mind: https://www.businessinsider.com/elon-musk-tucker-carlson-sex-procreation-collapse-civilisation-abortion-pill-2023-4