Collapsing is the word we use about the biodiversity collapse, or the collapse of some environmental numbers.
There's no catastrophy about having a population shrinking, we are doing that to most of the other species on the planet and only a few people seems to really care.
We are in a humanity re-adjustement, not a collapse.
Japan is already there.
South Korea is at one third the replacement rate.
This is the story almost worldwide, and soon can be expected to be worldwide. In China and India there is also sex selection abortions and male preference to make things worse.
Exponential on the way up, and on the way down.
Reversing such steep declines isn't possible in one generation without a "Marshall plan" for fertility, which means that in one generation the next cohort of fertile females will be half the size of today's if today's fertility rate is half of replacement. If the situation persists then the next cohort will be one quarter of today's.
These declines are much sharper than past growth rates.
What if these fertility declines persist for three generations? How many generations can they persist for anyways?
What goes up ends up to going down at one point.
We are reaching some limits, in a society that was used to think that we could push boundaries continuously endlessly for several centuries.
Declines are also exponential.
Plateuing could have been a thing. But instead we're now in a steep decline mode. It will be 150 years before populations increase again.
Probably many, as long as the growth rate was low enough, or zero. We can't grow forever, but that doesn't mean that the alternative must be to decline and overshoot whatever is a good population size (but alas this is what we are now bound to do).
SK though... yeah, something is uniquely broken there.
"Reversal" isn't possible with our without Marshall plan. What you need is young females and what you have is aging population, no amount of money or political will can change that. Even if we (I'm in Europe) manage to get out of decline, the recovery will take several generations which means a century of time.
South Korea is screwed way beyond repair by my estimates.
The current trend implies that the human specie is collapsing at a similar as the rest of biodiversity, it's not a readjustment.
We built an economy based on the fact that resources and population could grown indefinitely and now we are surprised that we cannot cope with limits. Looks like we'll have to learn it the hard way.
What's desirable is not population expansion but neither is population collapse, it's population stability.
This also has nothing to do with Capitalism being built for perpetual growth: of course it is deemed to fail at some point in a finite world, but that's irrelevant to the topic of demography. If anything, the system will see the population decline as an opportunity, as it means fewer people to share the resources with.
There is only one point of stability and getting there is like placing a basketball atop of a flagpole.
But now we are facing a crisis. For instance the next South Korean generation is set to be half of the current one. And fertility declined strongly there over the past 10 years. Many countries are now at levels SK had ten years ago (well below the replacement rate by higher) and if the trend continue they may end up in the same situation as SK in a few years.
Halving your population over one generation is a catastrophic event.
This is equally concerning as the collapse of the number of bugs. Some people can believe it doesn't matter, but the consequences are enormous.