It’s not obvious. Low birth rates are completely sustainable, you just kill anyone who can’t afford to retire and can’t work anymore, and society functions perfectly well.

The two biggest issues off the top of my head are rural towns in Japan will continue to lose population and completely disappear, and there won’t be enough young working people paying into health care and social funds to support the old non-working population. I think there are a lot of other major negative impacts Japan will face as a country but I’m just not that knowledgable on the subject.

I assume we just have fundamentally different views on this topic because I really wish humanity would change to a more scientific and explorative approach entirely, where we expand outward into space and become a multi-planetary species, which will need a huge sustained population growth to support. I assume you don’t support that.

Old people can’t work and need someone to pay for their retirement.

If there are more old people than young people (population pyramid wrong way round) every young person needs to pay a crapton of taxes so that old folks don’t starve to death