I think a lot of people are incorrectly identifying this as a good thing.

Really, this is an unprecedented phenomenon in recorded human history.

The entire social structure of basically every country on the planet is completely unprepared for the challenges that a rapidly aging popuation will bring. The effects won't be felt for decades after the change begins, and once it starts in earnest, it will prove very difficult to reverse in any short timescale.

It will become a sort of feedback loop - fewer people have children, so the capacity for children in the next generation is reduced. Because the retiree/worker ratio has now increased, the young people there are, are unable to afford having children, and so on.

Even with massive increases in productivity, you would see economies contracting. Ending up in a deflationary trap with increasing (and unsustainable) debt.

I see these kind of statements so often:

"AI will solve this problem" - really? The World Bank development report 2019 (https://www.worldbank.org/en/publication/wdr2019) basically comes to the conclusion that "while automation displaces workers, technological innovation creates more new industries and jobs on balance". This has been the case for centuries now.

Automation causing mass unemployment always seems right around the corner, yet never comes. I'm confident that we're now in the "trough of disillusionment" when it comes to progress on AI. Things like self driving cars for example are much more complex than we had considered.

"This is good for the environment" - It might not be though; the economic issues caused by aging populations could lead countries to take exploitative short-term decisions; converting rainforests to farmland, using cheaper means of generating electricity (that also prove to be more damaging to the environment) for example.

It is generally more expensive to be green (in the environmental sense).

The competition for immigration (which seems like an inevetable outcome in this scenario) could also destabilise certain countries, with potentially dire outcomes.

"It will reduce inequality" - Japan is ahead of the curve on the whole aging population front, and the average wages there have basically stagnated for the past two decades: https://tradingeconomics.com/japan/wage-growth even as the number of employed people has stagnated/fallen: https://tradingeconomics.com/japan/employed-persons

I think the keys to this are:
- Why are people having fewer children? If it's a genuine choice, then of course this is acceptable. But I think often the number of children people want to have is actually higher than the number they feel they are able to support due to economic (and other) reasons: https://www.economist.com/briefing/2016/08/27/the-empty-crib
- If the retiree/worker ratio is going to increase, and life expectancy is going to increase, then how can we manage this in a way that puts minimal pressure on health and social care systems?
- Are there any other ways we can prepare economically/socially for a society with an inverse population pyramid (I personally think this is a very, very difficult problem to solve, but would love to be proved wrong)
- How do we manage depopulation of specific communities? This is a problem that Spain is currently experiencing (e.g. it is more economical to forcibly move people away from communities that are dying out and no longer economically viable)