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.