It may work in short term but then you run out of population in long term. And suddenly any gains from short term improvement is under a big question mark.

Why does China need 1.5 billion people?

Future generations have to pay off the debt generated by the current generation. Same as everywhere. You can either grow the population or make the economy more efficient, China still has the latter lever available to pull.

China has a gross savings rate of 45-50%. Current gen has saved for their future.

Money is an abstract representation of the labor of others. If there’s no one to labor for you then your currency is just paper.

Currency might also just be bits on a server.

I agree in theory with you. But you can 'bank' labour for later on. Building infrastructure (that will last) when you have a population bulge, means that the generations after can take advantage of that infrastructure, and have to do the lower amount of work, of maintaining it. This can go for things like education, investing in businesses, and generally any capital.

Of course you can also over build this capital that will never be used, but will need to be paid back (for example a local government pays to build a town that there are no people to fill it with) and the maintenance is too much, but you still have to look after the people who did the building.

You can over-build things like old age care. We still don't have robots to feed and bath old people.

China has a poor social security net. The one-child policy has resulted in a couple having to provide or care for 4 parents and 8 grandparents.

That's is in no way sustainable on an individual level and worse on a societal level when you have a small working population and a large number of people that require care.

    strict population control

SG/KR has this? Tell me more.

Had.

A declining fertility rate doesn't just change change the population, but also the relative distribution of each age group. A fertility rate of "N" means the next generation will be "N/2" times as large as the one before it. Because of this you can easily model what a society, composed of successive generations of a fixed fertility rate, would look like. To keep things simple, imagine everybody gives birth to all their kids at 20. When they actually do doesn't matter, it would always converge on the same ratios:

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Fertility Rate of 2 (average age = 40): 100 80-year-olds, 100 60-year-olds, 100 40-year-olds, 100 20-year-olds, 100 newborns.

Fertility Rate of 1 (average age = 65): 100 80-year-olds, 40 60-year-olds, 20 40-year-olds, 10 20-year-olds, 5 newborns

Fertility rate of 0.5 (average age = 74): 100 80-year-olds, 25 60-year-olds, 6 40-year-olds, 1 20-year-olds, 0 newborns

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You can also estimate the ongoing population rate of change for each fertility group by removing the elderly generation, adding a newborn generation, and contrasting the new population sizes:

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Fertility Rate of 2: 500->500 = 0% population change per 20 years

Fertility Rate of 1: 175->77.5 = 56% population decline per 20 years

Fertility Rate of 0.5: 132->32 = 76% population decline per 20 years

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In the developed world most fertility rates plummeted around 1970. So, assuming nothing radically changes, we should start hitting our 'equilibrium point' about one life expectancy away, so sometime around 2050.