Immigration doesn't change the population, it just shuffles it around. The only ways to increase it are to increase births or decrease deaths. The article isn

Immigration doesn't change the population, it just shuffles it around. The only ways to increase it are to increase births or decrease deaths. The article isn't convincing that the U-shaped graphs are predominantly from increased birth rates. What is usually described is the pattern is birth rates decline as life expectancy rises, but the lifespan itself hasn't changed. Because so much of the life expectancy increase has occurred within the last couple of lifespans, and it hasn't globally reached the lifespan limit, we haven't seen population level off yet. But even when we do extend the lifespan, there will still be a death rate. And if the trend continues, the longer it gets the fewer births there will be. As we approach practical immortality, the dangers of catastrophes will be greater. That's the long-term population problem - when we only have one birth per year, and some disaster wipes out a million people.