The math is more dismal than that. With a 1.5 birth rate (common in Europe) the population drops by more than half in three generations (about 100 years). This causes the economy to collapse—stock valuations have baked in assumptions of future generations of consumers existing. But it can get worse. South Korea has a fertility rate below 1 and Taiwan is headed that way. At that rate, the population drops by almost 90% within a century.