No overpopulation. They conflate life expectancy with fertility.
Pooley and Tupy, 22—associate professor of business management at Brigham Young University-Hawaii; senior fellow at the Cato Institute’s Center for Global Liberty and Prosperity (Gale and Marian, “Julian Simon and the bet that made him famous,” Superabundance: The Story of Population Growth, Innovation, and Human Flourishing on an Infinitely Bountiful Planet, Chapter 3, 111, dml)
First, as Ted Nordhaus from the Breakthrough Institute in California observed in 2019, the frameworks assume that humans will continue to reproduce with abandon. In reality, data show that as an economy becomes more developed, birth rates begin to fall. The massive population growth in recent decades is due to rising life expectancy, not rising fertility.49 The birth rates of the United States, Europe, Japan, China, and large parts of Latin America have actually fallen below the replacement level of 2.1 children per woman for some time. That means that their populations are actually starting to shrink. It’s likely that the rest of the world will follow that trend. What does that mean?
According to Bailey, the “world population will likely peak at 9.8 billion people at around 2080 and fall to 9.5 billion by 2100 in the medium fertility scenario calculated by demographer Wolfgang Lutz and his colleagues at the International Institute of Applied Systems Analysis. Alternatively, assuming rapid economic growth, technological advancement, and rising levels of educational attainment for both sexes—all factors that tend to lower fertility—Lutz projects that world population will more likely peak at around 8.9 billion by 2060 and decline to 7.8 billion by the end of the twenty-first century.”50