What matters is not one's own babies. But "more babies".
The more wealth created, the more babies can be supported with the wealth. Musk in particular has created many high paying jibs that are undoubtedly anchoring solid families with strong economic foundation.
For Musk, its a prohibitive opportunity cost for him to have babies of his own vs enabling hundres of others doing the same.
Musk will not be the face of a fertility campaign. Musk is not the paragon of virtue. But he's helping support tons of families have their babies
The guy can almost field a basketball team with his kids from a string of different women. He obviously has no qualms having kids. Raising them on the other hand, seems to be too much of an opportunity cost to the self-proclaimed titan of industry.