[Secret Agent Man]

Um, there’s no requirement that the population can never go down.

[who_would_fardels_bear]

It's not just that the population will go down, but that an increasing percentage will be old. That's a bad mix for welfare states like every "capitalist" country on Earth.

Also, more economists are suggesting that the economy is something of a pyramid scheme which requires ever newer, younger, bigger populations to increase demand. As people age, they buy less. Stocks will go down. Real estate will also. So if old age pensions go away, even the pragmatic folks who plan ahead will have little to invest in.