You need people who can actually do work to take care of all the old people & sustain human society. “Less People” is not by-default “more sustainable” especially not if it happens all at once; that was in fact a huge problem with cyclical famines & political turmoil in the days before mechanized agriculture.

If some asshole went around raiding hamlets for plunder, or whatever reason, yeah that would mean fewer mouths to feed in that particular area, but it also means fewer hands to bring food to harvest. Which means other regions have to contribute larger proportions of their own food-stock to sustain the needed intake of urban centers. Which means that they have less food to eat for themselves, and less to replant for the next harvest. Which pushes people on the margins of the the agricultural economy into banditry to sustain themselves, which causes us to return to the beginning of our story.

Eventually this cycle of regional depopulation leading to productivity shortfalls, leading to further regional depopulation becomes self-reinforcing & before you know it you have a country-wide catastrophe on your hand & the total implosion of existing society.

Now we aren’t dependent on mass manual agriculture these days, so famine specifically is an unlikely cause of cyclical societal collapse, but the modern world still requires that a shitload of manual physical labor get done in order to maintain the basic infrastructure that gets everything from where it is, to where it needs to be in order for us to all not die. If you don’t have people to fill those positions, then that’s work that needs to get done, that isn’t being done.

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

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.

“Unsustainable Society” No matter your opinion on current governments, humanity has been around for an awful long time, and it will likely continue to be around for significantly longer into the future of the universe. In my opinion, that’s pretty cool.

In the grand scheme of things, just looking back over the past couple hundred years, the vast majority of humanity is in a better spot than we were, no matter how bad things may seem on a small time scale.

“Has almost made the planet uninhabitable” The Earth is definitely worse off since we have proliferated, but this is such a clickbaity untrue statement.

Humanity has and will continue to cause changes to the world that are negative, I agree, and that sucks. But like it or not, humanity is good at adapting and surviving, and we will be fine, even with the worldwide population overall continuing to grow for a very long time into the future.

I just disagree on the infinite growth being unsustainable thing. Humanity, in my opinion, is destined to expand to the stars where we will continue to grow Indefinitely on a time scale that actually matters to you and me.

Obviously, that could not happen if we somehow all die, but despite all the doom and gloom, I really don’t think that’s likely.