Most countries social security systems rely on workers today paying in.

The dependency ratio of workers to people on pensions may get to the point where it breaks.

https://fred.stlouisfed.org/series/SPPOPDPNDOLUSA

Yeah, but more worker doesn't automatically mean more money for pensions.

What matters is productivity and the workers share of it.

Fewer workers with higher wages have the same effect on pension payments.

Pretty much hot topic in finland right now

Aging population, lower economic growth (less consumer demand for goods and services).

Aging population per se isn't a problem, because better hygiene, lower child mortality and enough food creates the same effect.

Lower economic growth doesn't have to be. Fewer people means less supply on workforce means higher wages means higher purchasing power. The existing people would like to be more, but can't afford because of low wages. Fewer people will give them leverage.

Higher purchasing power per person not overall. That would increase the wealth inequality divide and the aging population would suffer as lower and middle class services struggle in favour of high wealth services. We are seeing that in clothing at the moment, high end clothing is doing really well but serves fewer people.

Japan is a country to watch, it has an aging population but historically good support for middle class services through lots of small businesses. If that changes for them it is a bad sign for the West, as we generally have greater wealth divides to begin with.

I'm talking about workers, they are mostly middle and lower class. Your example of higher clothing, it's not the workers who do really well, it's the designers and labels. Wouldn't consider them as workers.

Less pollution, less global warming, lower use of resources, less conflict.

Counties with declining populations tend to become industrial catastrophes.

Japan and Fukushima come to mind. Russia does too.

Russia and Ukraine both have abysmal fertility rates. I don’t see less conflict there. Or between China and Taiwan.

I doubt that that's a causality. Japan isn't a catastrophe compared to any country with a rising birth rate.

And Fukushima was an accident caused by a natural disaster and human errors. More babies wouldn't have prevented that.

Because it's exponential decay: population_n = population_0 * 0.55^n (made-up exponent).

In any oscillating trajectory you’d see exponents above and below 1. Tomorrow’s birth rate is not impacted by today’s. But the fear is that today’s birth rate will continue forever until societal collapse, which seems unlikely.

Extinction of peoples, nations, cultures and civilizations is not trivial. At least for normal people.

8 billion people isn't extinction of people and nations, cultures and civilizations went extinct even with and sometimes because of more people. Resources are limited and more people fighting over them leads to war which leads to extinction of nations, cultures, civilization and people.