system
Given the following post, respond as if you're commenting on Reddit:
human
How is population decline solved on a global – not country-by-country – level?

Immigration is commonly mentioned as a solution to population decline for a country, in relation to maintaining the current economy. But how would immigration be addressed on a global level, as those who immigrate must emigrate from somewhere?

* in the long term, would one or a few countries be regarded as ’immigration pools’ by which other countries solve their own population decline?
* since population decline is associated with improved health and conditions, would the aforemention ’immigration pools’ have to be countries with worse health and conditions?
* if population decline happens in all countries, will immigration be driven mostly by countries competing with alternative benefits for those who immigrate?
* if yes to the previous question: in the very long run, what would happen to those countries that can’t compete with attractive immigration benefits?
gpt
I have never understood why many people think of population decline as a problem.

The whole point of the original Becker model is that there is a trade off between quality and quantity of kids. The same thing happens at a global level. With scarce resources, a smaller world population means more resources per capita and, potentially, higher quality of life for each person. 

I understand that any demographic transition poses short-run problems because the existing institutions were designed with a specific demographic composition in mind. This is where immigration can help. 

However, in the long run, population decline is not necessarily a problem. The world population is projected to stabilize or peak around 12 billion people within the next few decades. Who says that is the “optimal” level of world population? Maybe the optimal level (according to any criteria you can think of) is less than that (maybe greater). Who knows.

Perhaps when you think about population decline you are worried about the extinction of the human race of something of the sort. But you must remember that the growth rate is not an exogenous variable. In a world o with smaller or declining population, having more kids will become more attractive and fertility rates are likely to increase.