The dependency ratio of workers to people on pensions may get to the point where it breaks.
What matters is productivity and the workers share of it.
Fewer workers with higher wages have the same effect on pension payments.
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.
And Fukushima was an accident caused by a natural disaster and human errors. More babies wouldn't have prevented that.
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.
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.