I think you oversimplify this. Long term decline in population would be fantastic (as long as it tapers off before it hits zeros!) but precipitous drops in birthrate are indisputably hard on economies. People disagree about the first part of that, but no serious economist doubts the second part. The problem is making fewer babies changes the distribution of ages. Take Iran. In one generation, the fertility went from more than 7.5 children per woman to 1.6. It's not bad at first, because you have a huge number of people of working age, with relatively few children and old people. But throw in an increasing life expectancy from modernization, and down the road you get this giant bulge of aging people with few young people coming behind them to pay their way. This makes countries quite poor. (E.g Ireland of 1850 to 1960) Today, many countries have growing populations, but in almost all of them, it's because they developed so fast that they still have a disproportionate percentage of the population in their baby making years. If you normalize that out, there are almost no countries in the world that are at replacement (app 2.2 children per woman.) I, personally, think there are about 6 billion too many people in the world, but the process of going from what we have today to a sustainable population of a billion or two will be by far the hardest on the poorer countries, because they have the largest number of young people today. The young will get old faster than those countries will get rich, so they're looking at a 100 years or so of difficulty until the demographics level off.