[Question] Is population collapse due to low birth rates a problem?

Elon Musk published a few hours this tweet:

“Population collapse due to low birth rates is a much bigger risk to civilization than global warming”

Robin Hanson, who is something for whom I feel a lot of intellectual respect, liked the tweet.

In my model of the world works, overpopulation is in fact a big problem. In general, the more people you have, the less resources you have to share among those people. A decreasing population would be in fact good news, although maybe not in the short term.

Can you help me understand what are Elon/​Robin seeing that I am not?

A couple of extra points for the sake of clarity:

  • I do understand that, in the current system, having an aging population is a problem because many resources go toward people that reach an old age

  • AI might or might not end the world. Let’s assume in this scenario that it does not and we have many more decades ahead