I disagree - declining population is a disaster that only ends in a country becoming poor. As you allude to in your response, sub-replacement-rate fertility is what drives population decline, so declining populations are always ageing populations. The economics of ageing populations don't work for obvious reasons - as people get older they need more care and produce less, and with fewer workers to provide that care, the care becomes more expensive in ways that governments never provisioned for (see Japan and many countries in Western Europe for good examples of this in practice).

In the longer term this depresses GDP per capita as resources have to be diverted from productive industries like high-tech manufacturing into extremely unproductive industries like aged care. Again, Japan and many Western European countries provide good examples of this in practice. Finally, as the reality of the broader economic decline sets in, second order effects start to take hold like brain drain - as young workers see higher wages working in more productive economies, they decide to leave, worsening the population pyramid and depriving the government of precious tax revenue needed to pay for aged care. (Japan and many Western European countries are sadly, again, examples of this phenomenon).

The short term effects you're describing of nominal GDP rising and per capita GDP falling is only true when populations are growing through bad immigration policy, which is what we commonly see in the US for example. Growing population organically and sustainably, or through productive immigration that targets gaps in the country's labor force, should never have that effect as the new member of the population should eventually increase GDP by more than the average existing worker today.

At an individual rather than population level of course, having fewer or no children is easier, which is why people are doing it. But this describes a local optimum only, while the true optimum is population growth.

I think you made an erroneous assumption. Here you say the declining birthdays are always aging populations that require more care and this is a net drag. First, care and health services do contribute to GDP and provide good income work for those providing care. Second, while the aging population is larger than the younger population who are working at first that’s just because during the elder generations birth rate was higher. As that ages out of the population you’re back to an equilibrium. Third, aging population doesn’t mean decrepit population. Part of our aging story in the developed world is that quantity of life has sort of capped out but we are continuously improving the quality of life at all ages through medicine, nutrition, education, etc. Fourth, people are working considerably longer. People used to retire around 50 to 55, then 60-65, now it’s more towards 62-70. I don’t see that trend reversing frankly especially as quality of life care improves which I expect it to dramatically in the next 10 years. I suspect we are also within spitting distances of genetic, mRNA, and other cancer treatments that virtually eliminate the need for radiation and chemo while providing astounding outcomes as close to “cured” as you can get with cancer. Fifth, you bring up Japan as well one should - but if you look at Japan they’re doing fine by any standard and their per capita GDP is about the same as it was in 1990 in adjusted currency. It’s not like we’ve never seen youth / aging boom and busts in history and everything is fine.

As a final point I think older workers are generally more productive as long as they’re able to use the tools they’ve developed skill with. With the advent of computers and information technology older generations were unable to map their skills to the new way of work. Society existed in the state they were used to for 10,000 years and suddenly instead of being masters as the elder had been, they were bumbling fools - made more tragic in that they were otherwise highly skilled at what they did with a lifetime of experience. This colors our perception of the aged because we are still in the middle of that transition. But do you think you and I will be limited by ability to use information technology? Mental decline doesn’t generally start until much later in life, and people are doing careers into their 70’s with success.

>while the aging population is larger than the younger population who are working at first that’s just because during the elder generations birth rate was higher. As that ages out of the population you’re back to an equilibrium.

This is absolutely wrong. The "aging population" will be larger than the "younger population" in any situation in which there is sub-replacement fertility + stable (or increasing, obviously) life expectancy. It's just that the ratio may stabilize but it will be <1.

I think you're thinking about 'old professionals' when whats under discussion is "jesus fuck you are old as dirt" people, and how the economics change when your distribution starts skewing towards that old and ineffective side.

> while the true optimum is population growth.

Right up until you overshoot your ecological carrying capacity and the whole society collapses, of course. That part tends to be less optimal.