They are concerned with survival of civilisation.
Its not "according to some powerful people"; it is basic maths and biology. If fertility continues at well below replacement level in civilised societies those societies will die out.
It has happened many times before in history (Greek Dark Age, Collapse of Rome and Dark Ages). But this time it is happening to almost all civilisations at once - because culture has gone global.
hey bud, it's great to see you are still around.
Well, I'm glad you are pushing back on my point because this is the way we all learn, by having these frictions.
it seems to me that the population of the world keeps on growing and exponentially so. I take the concern can be, and maybe this is your case: Who is doing the growing?
If this is the concern, again one that I'm not dismissing. Who is to say that these "new peoples" as I'll call them for simplicity are not going to be the new consumers and producers of resources.
On the other hand, I do find it hard to believe someone like Musk is speaking from a place of love for humanity. It would be out of character with everything I've seen him do and say.