I noticed that this tweet from Elon Musk: this story:
One, the key to raising children is not being the sperm donor, which while a necessary step is far from a sufficient one. The real effort is the bearing of the children and taking the time to properly do tasks like childminding and educating children. One rich person having children with many women (Musk has 10 children by 3 different mothers) is not really doing much to address the fertility crisis.
Two, Earth reached a population of one billion people around 1804 CE. That is 218 years ago. Recorded history goes back to at least 3200 BCE. That means we had 5,000 years of successful human civilization with less than a billion humans. I think we can start getting worried about underpopulation when we hit one billion going the other way. Until then, I think concerns about whether there are enough people seem odd.
Finally, having children with a subordinate at work seems like a suboptimal decision. It is hard to ensure that there was a proper balance of power in the relationship. While the person in question, Shivon Zilis, seems to have a lot of agency in their career, it is always a concern in these cases.
So I would say that this strategy was not the optimal way to handle concerns about a population crisis. It might be fine for Musk and he likely has the resources to support this eccentricity, but I think that is what we need to call if it we are honest.
There are good pro-natalist policies that could be considered. My personal hint is that there is no way to understate the importance of childcare and that a positive childcare experience probably helps people match the number of children they prefer with the number of children they have. But I think these should be considered in the context of human happiness and not because we think we might be going extinct as a species.