The more interesting thing about this is that it is part of a new type of economics, contraction economics. The global trend toward birth rates below replenishment means that first-world countries are maintaining their population growth through immigration. As some countries embrace restrictions on immigration, and ultimately all countries plateau in their population growth (projected to be around 2100) it means that…
Well, this is all assuming that humanity will not expand out of Earth. If that does happen, we might actually have a perpetually expanding populace and economy. Fun times.
Population might just stabilize at some point, and then there's not much of a driver to expand off of our planet (apart from some resource mining, and other activities.)