He seems to be focused on the developed nations, which do have declining birth rates. Those countries with high birth rates are mostly supported and even directly fed by the developed nations. At some point there will not be enough smart people with high resource demands to support the industries that feed almost everyone in those poorer nations. As our populations slowly collapse so will theirs. I like that Elon is saying we need to have more babies. His solution to this problem is not using the undeveloped world as a source of new citizens. Flooding developed nations with people who are not mentally capable of doing complex, modern work will only bring on a faster collapse. He seems to understand that our civilization is not the slums of poor nations. Those are an offshoot of our civilization. If the developed nations collapse our civilization collapses.