Some kinds of babies are good and other babies are bad (in their view). They only want "good" babies.
Trump's unconstitutional executive order targeting birthright citizenship tells you all you need to know about who they think the bad ones are.
And though many have tried, no country has figured out how to meaningfully drive up birth rates with pro-natalist policies.
There's only one proven way to get more babies and sustain economic growth when native birthrates decline: MORE IMMIGRATION.
If immigration to the US continues apace, we surpass China's total population within decades and continue strong economic growth comfortably. If TrumpElon really manage to kill the golden goose, welp... it's been real.
First, I'm saying Musk's apparent interest in birth rates is best understood as a component of the neo-fascist, authoritarian project in which he's embedded himself, the likes of which we've seen many times throughout history, and is the kind of thing that often culminates in large scale atrocity.
Second, I'm saying for anyone who is truly interested in driving population growth, the last thing one would do is inhibit immigration. Instead, you'd open the floodgates.