Yes, sure, but! in order to save you also have to have people who will later be willing to provide goods and services in exchange for your savings, and that req

Yes, sure, but! in order to save you also have to have people who will later be willing to provide goods and services in exchange for your savings, and that requires having children. No, I don't mean that everyone must have children, but that in a world where population pyramids are inverted, older people will have a hard time getting much value for their savings. Taking this to an absurd extreme, the last human alive could have all the money in the world and it would do them no good at all -- all that money would have exactly zero value (well, the cash could be used as fuel or toilet paper, but that's it, and most money isn't and wouldn't be cash anyways). Now, a society might be able to survive having an inverted population pyramid for a while, or even a very long time (generations) but e.g., dedicating more of its economy and people to elder care, by automating more of elder care, and so on. So it's exactly not all bad. Nor am I saying that fertility rates should be above replacement rates -- we all know that we can't have exponential population (or even economic) growth without end on Earth. But do note that an economy with a long-term inverted population pyramid tends to be inflationary: the elderly will have a lot of money (as usual), yes, but it will be chasing a smaller population of service providers. Talk about Childhood's End. Sustainability yes, self-destruction no. In the long-term I do assume -perhaps wrongly!- that countries with negative population growth will eventually stabilize their fertility rates at replacement rate (probably with an overshoot on the upswing).