Underpopulation itself is not a problem
Low fertility will lead to population extinction.
Underpopulation in certain countries could cause undesirable demographic shifts
An aging population will cause the social state to overload.
These are the main concerns I see with low birthrates - let me walk through them all.
Low fertility will lead to population extinction
This is almost certainly false, and it’s false because population density and fertility have a strong causal relationship. Fertility increases density by creating more people, and density causes lower fertility because it increases competition for zero-sum resources. If this wasn’t true for animals, their populations would be extremely unstable - in practice they tend to stagnate.
I tried replicating the results of the study I cited using data from ourworldindata, and calculating the correlation between density and fertility within countries:
Looks pretty close - I tried extracting the effect of the year using the partial correlation, and got these results:
The average correlation increase from -0.8 to -0.35, probably because the fertility decline and population density increases are confounded by time.
Spandrell brought up an important point, which is that even if density is causal for fertility, people immigrate to high density areas. If this is the case (and I know it to be so), then the fertility reversal may happen later than expected. I have no problem with this in terms of values, as I support depopulation, but it could be an issue if the fertility rates crash so hard that the West can no longer feed its parasites.
I forgot to mention that Karlin argues that undending population collapse due to self-selected low fertility is impossible because fertility is heritable (10-20%), so at some point this psychological profile will become selected for and reverse the decline. Of course, there is no guarantee that genes that cause fertility in one environment generalize to the rest, but if there is an significant environmental shift in regards to fertility this probably implies some kind of change in terms of average fertility within the whole population
Undesirable demographic shifts
You can’t increase fertility rates. Money doesn’t work. Propaganda doesn’t work. Telling people to “have kids” on twitter doesn’t work. Believe it or not, giving teenage girls dolls does work, but I’m not sure if increasing births in this age cohort would even be desirable, as early births tend to be from people low in intelligence.
But guess what you can do? Just close the border.
High birth rates in Europe + third world + immigration would hardly be better than the status quo, if anything that may actually would be worse as developed countries would get overcrowded because of the combination of immigration and high native birth rates.
There is also the question of non-White immigrants outbreeding natives - before this did appear to be a large concern, but controlling for immigration status causes fertility differences between natives and non-natives to reduce in some countries, notably America and Denmark. The fertility differential between immigrants and natives is also not observed in all countries.
Aging population causing financial collapse
Note that if people’s income varies by age, if the nation becomes older, the incomes will reduce, but independent of government transactions, people’s lifelong income and living standards would not change on average. So the problem here is not just the aging population, it’s the combination of the aging population and the welfare state that is.
Unlike closing the border, cutting back on pensions unfortunately is a political non-starter, even if the population were to awaken on this issue, the financial and voting power of the old is too strong. As far as I can tell there is no solution to this problem besides hoping that AI takes off and the need for labour decreases. And even if population aging were to cause pensions to reduce, it wouldn’t be that much of an issue - most old people are wealthy anyway, and are at the end of their lives. It would be bad, but probably not civilization ending.
I think eugenicists often fall into the trap of thinking more younger women giving birth might be net negative because of the negative IQ correlation associated with younger births/teen pregnancies. The reality of more births to younger women in a declining population is that you get more births of high IQ people overall at the possible expense of a couple IQ points per century or half century. Meanwhile you also get to offset some of the population decline. Seems clearly worth it to me. After all, as you point out, the looming pension crisis seems nigh-unsolvable without ASI.
Aging populations will demand a lot of resources for the young. You note this is bad but not third-world immigration bad. But I think it might be: because intelligence decreaes with age. You have half a population whose intelligence will decline by a standard deviation and you have the equivalent of millions of African-Americans--except need even more health support. This will simpyl stagnate and recess societies.