When Mr. & Mrs. Singapore and their age-cohort are old enough to retire, they will have only one offspring (per two retirees) to support them.
That one adult child of their will be able to find a spouse, but the two of them will have 4 elders to support. (Don't talk to me about social security. It's paid by the working, taxpaying population.) The working-age generation may want to have children of their own, but on the other hand they may want to put it off, tax-burdened as they are by an ever-rising percentage of dependent elderly and super-elderly.
So things are likely to get worse. Many couples will not reproduce at all. And the upshot is:
"The government, fearing that the tiny, prosperous city-state will shrink into oblivion, has recently adopted a series of incentives to encourage people to have not just one or two but three or more children.....Singapore's birthrate has sunk to an all-time low... Raising it has become a national cause, as significant as the fight against terrorism. If the birthrate continues to wane, officials warn, the workforce will shrink. There will be fewer people to support a growing elderly population and to sustain the military that protects this 400-square-mile island sandwiched between Indonesia and Malaysia. Singapore's vaunted tiger economy will whimper.."
You might want to look up the news article these quotes came from to get a real-life perspective on Singapore's no-future future.
I don’t need to do the math. In a nation of declining birthrate (that is their option, don’t you agree?), the nation has a choice of saving more for later years or receiving less in old age pensions or welcoming immigration. That is their choice. It has nothing to do with math. It has to do with choice. With the planets population, no nation will go extinct due to low birthrate. It’s just not possible.