The saying "demographics is destiny" encapsulates the reality that demographics--rising or falling trends of births and deaths--energize or constrain economies and societies regardless of other conditions.

Demographics are long-term trends, but the trends can change relatively rapidly while policies remain fixed in the distant past. This disconnect between demographic reality and policies has momentous future consequences. An appropriate analogy is the meteor wiping out the dinosaurs; in the case of demographics, this equates to the complete financial collapse of the retirement and healthcare systems.

As this article below mentions, extrapolating the high birth rates and falling death rates of the 1960s led to predictions of global famine.

As death rates declined and women's educational and economics prospects brightened, birth rates fell, a trend that now encompasses most of the world.

As a result of the Green Revolution (hybrid seeds and hydrocarbon-based fertilizers), the Earth supports more than twice as many humans as were alive in the 1960s (3.5 billion then, 7.9 billion now).

Now the problem is a shrinking working-age population that will be unable to support the financial and healthcare promises made to the retired generations.