Japan has only just begun its implosion. Population dropped by about 0.5 million last year but that will 3x within a decade given their population pyramid.
Losing 1-2% of your population per year is catastrophic and exponential.
Totally different circumstance because the population pyramid even after black death looked great. Tons of incoming young people every generation. In fact it improved the pyramid! The black plague disproportionately killed old people.
This is going to end up in a top heavy pyramid with a ton of old people relying on fewer and fewer young people. Simply because fewer and fewer are being born every year.
Losing 1-2% of your population per year is catastrophic and exponential.