Why is everybody consistently wrong about this subject on the Internet? Developed countries simply do not have a birth rate high enough to sustain population size. Is the birth rate currently too high in certain developing nations? Yes, but that is a direct result of the economic status of those countries, and birth rates fall as economic growth grows. There is a reason that countries are desperately trying to craft policies that lead people to have more children; the birth rate is far too low. Populations are aging like mad because there are fewer younger people being born in each generation, and only mass immigration is saving these countries from ruin as the elderly retire. As world incomes become more equitable and borders tighten, this will no longer be a sustainable solution for many countries, especially as anti-immigrant reactions will rise as people feel like foreigners in their own countries. Please, let's actually look at the facts and stop this nonsense over unfounded worries. The world birth rate does not reflect the trends in the developed world, and as the world develops, decreasing family sizes will bring the rest of the world in line with these trends.
Also, even if this were a problem, this solution is morally disgusting, but other people can handle that here.