(The USSR had an above-replacement fertility rate under Stalin. Population and GDP growth is not exactly a capitalist thing.)
Barring anything better, we're stuck with the null hypothesis: getting out a ruler and extending the trend line into the future.
If the population is growing, then eventually Japan would have billions of people where there are currently hundreds of millions. There was much speculation in the 50s on how you could deal with that scenario if it happened, but it's not looking very likely now.
If the population is exactly static, one person born for every death: not too bad. You could call it "stagnation" if you wanted to be negative.
If the population is declining: it will eventually hit zero, and the country will cease to exist. Japan's population declined by 1.8 million people between 2010 and 2019. If the decline is linear, (which it won't be, considering their age distribution) then that has them hitting zero in 2089! If you think that scenario won't happen, then please provide your reason. "That would be terrible, so it won't" isn't a reason.
The UN population projections show European and North American fertility rates leveling off and even increasing. That would be very good news, if true! Fortunately, they explain their reasoning: https://population.un.org/wpp/Publications/Files/WPP2019_Hig...
>The fact that fertility preferences remain close to two children per woman, even as realized fertility has fallen well below that level, suggests that the fertility rate in low-fertility countries may increase as populations learn to manage and mitigate some or all impeding factors. The trend in many countries towards gender equality and women’s empowerment, as well as expanded access to sexual and reproductive health care and services, also indicates that it may become possible for more women and couples in current low-fertility countries to achieve their desired family size, eventually raising the average fertility levels. Given that a growing number of countries is expressing a desire to increase the fertility rate and that some have achieved this outcome, supported by government policies and programmes, a rebound in the fertility trend in current low-fertility countries seems the most plausible future trajectory over the long run.
I am not seeing where this optimism is coming from. A graph of fertility rates for a basket of European countries shows a pronounced sub-replacement horizontal trend, with no sustained growth or decay. https://data.worldbank.org/indicator/SP.DYN.TFRT.IN?location... This foretells a gentle decline, rather than the sharp crash that's apparently in the cards for China, Japan and South Korea; but provides absolutely no evidence that more "reproductive health services" will rescue us.
I think it's quite daft to say gender equality leads to higher fertility without further explanation. Gender equality leads to a much higher participation of women in the workforce. At least naively I would assume this correlates with a lower fertility since women now are forced to choose between raising a child or having a career.
The only way I could see this changing if men would take on much more of the caregiving role. But that is not currently happening.
