And a population 'collapse' is not necessarily bad for a country or a region in the long term. Indeed it can be rejuvenating in the long term. Europe suffered a massive population 'collapse' after the Black Death pandemics. This had the long term effect of making peasants' labour more valuable which ultimately led to the Industrial Revolution and the rise of a Middle Class.
And raw population numbers are the least of the problems here. The comparison to the Black Death is rather silly.
Why? It's a very well explored and prominent example of a population 'collapse' leading to long term rejuvenation (and innovation).
I'm not pretending that some of the short term to medium term consequences of a population 'collapse' (I keep putting it in quotes because it's your word and I think it's too emotive a word) won't be dire. But at some point a population has to go through a period of decline. We see this all the time in the natural world. People who don't want to have any periods of decline and go all Pollyannish with emotive talk of 'collapse' and worse for any period of population decline seem to be stuck in a fantasy world, in my opinion. Not the real world, where, to slightly belabour a point, animal populations go through cycles of growth and decline, repeatedly.
And the answer is that there will be something like two 80 yr olds for every 30 yr old.
Population decline resulting from decades of abysmal fertility rates is absolutely a horrible thing.
When people stop working, they stop contributing to society but they don't stop taking from it.
For as long as human society has formed, the solution was either the working population being productive enough for everyone else or the non working dying off soon after the fact.
When fertility rates are in the shitter for a long time, you guarantee that at some point, the working population will be far far lower than what is necessary to support the rest.
A demographic and societal collapse will ensue. Many benefits and safety nets for many people including the working will cease to exist simply because it can no longer be sustained. Once this hits in full force, it will last decades. It's that simple.
And it has nothing absolutely nothing to do with capitalism or socialism or communism or whatever -ism you might want to scapegoat. For as long as humans need to work to survive this will be a problem.
The problems Japan is having, South Korea is looking to speedrun. It's so bad that it's too late to forestall a collapse. Even if the country magically started having replenishing birth rates immediately, in at most a few decades, we'll be seeing a collapse that would last at least 20 years. This is the best case scenario. It's straight up apocalyptic.
This is what a population table looks like for a country that had a fertility rate of 6 at year 0 and then an extinction level rate of 1 for decades (SK is actually decidedly worse at 0.78 now).
(148) Year 0: 4 sixties, 11 forties, 33 twenties, 100 newborns,
(194) Year 20: 11 sixties, 33 forties, 100 twenties, 50 newborns
(208) Year 40: 33 sixties, 100 forties, 50 twenties, 25 newborns
(187) Year 60: 100 sixties, 50 forties, 25 twenties, 12 newborns
(93) Year 80: 50 sixties, 25 forties, 12 twenties, 6 newborns
(46) Year 100: 25 sixties, 12 forties, 6 twenties, 3 newborns
(23) Year 120: 12 sixties, 6 forties, 3 twenties, 1 newborn
It's like climate change. You go decades with everything looking normal at first glance. By the time, everything doesn't look so normal it's to late to do anything other than save your population from extinction. (Assuming you can even do that. A lot of people seem to miss the fact that not one country that has suffered declining fertility rates has managed to reverse the trend despite trying for decades. Not one)