Scott Alexander Siskind, aka "Astral Codex Ten guy", weighs in on underpopulation concerns.
"1: Declining birth rates won't drive humans extinct, come on." "Not only are we not going to go extinct because of underpopulation, population is going to continue to rise for the next 80 years."
"2: Immigrant-friendly countries will keep growing." US will go from 330 million to 430 million in 2100. UK will go from 60 million to 80 million.
"3: Countries with low immigration will shrink, but mostly slowly." Brazil 210 million to 190 million in 2100. Germany 80 million to 70 million. Japan 125 million to 70 million. India 1.3 billion to a peak of 1.8 billion in 2060, then down to 1.6 billion in 2100. China 1.4 billion to 800 million in 2100.
"4: Big relative drops still imply high absolute populations." "But Japan and China will drop a lot. By 2100, there will only be 800 million Chinese and 70 million Japanese."
"5: Concerns about 'underpopulation' make more sense as being about demographic shift." "In high-immigration countries, declining birth rates will cause changing ethnic demographics, as native populations shrink and immigrant populations increase."
"6. Age pyramid concerns are real, but not compatible with technological unemployment concerns." "It's weird to be worried both that the future will be racked by labor shortages, and that we'll suffer from technological unemployment and need to worry about universal basic income."
"7: Dysgenics is real but pretty slow." "Another potential demographic shift in both types of country is shift among social classes / levels of educational attainment."
"8. Innovation concerns are real but probably overwhelmed by other factors." "Consider the century 1820-1920. It gave us the steamship, the railroad, the automobile, the factory, mass production, electricity, refrigeration, radio, the airplane, etc, etc, etc, with a population only about 10-20% as high as today."
"9. In the short-to-medium run, we're all dead." Alrighty then. Wait, one more...
"Appendix: The Amish inversion." "Suppose that there is a 2100 -- and even a 2200, 2300, etc. what happens if we extend current trends? Answer: the Amish take over the world."