I don’t actually get why this is a problem. We’ve got 8 billion people right now. We don’t need that many people to run the economy. As new tech continues to develop we get more productive. If we steadily grow in productivity we can produce the same economic output with fewer people. The income and wealth per capita goes up. In the case of Japan their GDP is flat but the the Japanese people are better off because that same GDP is spread out to fewer people. 

With the environmental impact of humanity, it’d be a lot lower with 2 billion instead of 8 billion people but we could have higher quality of living. 

And isn’t this the best way to reduce population? Slowly through people having fewer kids? Want to have a lot of kids. Go for it. Most women are choosing fewer kids as they get more economic and social power. It’s a hell of a lot better than China’s one child policy, disease, war, or famine. 

Yes we need to pay people to take care of the elderly. But we could do that the way Singapore does it. They don’t tend to put the elderly in institutional care. Instead they pay family members to take care of their elderly. Just like raising kids in a family is be than an orphanage, or other group child raising like happens in some christian communes and kibbutzes. Most people provide better care of their parents and grandparents than any nurse in an institution could. And it’s a hell of a lot cheaper, it’s just there is no companies pushing for these polices like there is for care homes. 

We can handle massive reductions in population over a few generations humanely without destroying the economy.