It's always interesting to look at what these numbers exactly mean because it's deeply counter-intuitive. For the sake of simple modeling, imagine we have a society with 100 people with a fertility rate of 1, that give birth at 20 and die at 80. Here is how that looks:

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Year 0: 100 newborns

Year 20: 100 twenties, 50 newborns

Year 40: 100 forties, 50 twenties, 25 newborns

Year 60: 100 sixties, 50 forties, 25 twenties, 12 newborns

Year 80: 50 sixties, 25 forties, 12 twenties, 6 newborns

Year 100: 25 sixties, 12 forties, 6 twenties, 3 newborns

Year 120: 12 sixties, 6 forties, 3 twenties, 1 newborn

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In spite of having an extinction level fertility rate, the population nearly doubled in the first 60 years, going from 100 to 187. And it took 80 years to even see the population begin to decline. But then suddenly over the second 60 years, the population exponentially declined going from 187 to 22. This is very akin to the scenario in Korea, because they went from a fertility rate of 6+ to < 1. So they're starting with a large "newborn" population.

Because of the fact that we live much longer than we are fertile, it really damages any idea of "Well we'll just solve this when it becomes a problem." When it starts to become obvious there's a problem, the decline is already coming at an exponential rate. And it's entirely possible that such a small youth population supporting a suddenly massive elderly population will drive fertility rates even lower.

It increasingly seems that the future of our planet will not be decided by politics, ideology, or anything of the sort. It will simply be decided by whichever groups have children at healthy rates.