Immigration is the obvious solution to this problem. (not claiming its the best, mind you) If the fertility rate is around 1.7, then the country should accept

Immigration is the obvious solution to this problem. (not claiming its the best, mind you) If the fertility rate is around 1.7, then the country should accept around 20% of their population in immigration over a generation to achieve a stable number. Assuming a generation is ~30 years, that's around 60 million new immigrants in the US. Assuming the immigrants are from underdeveloped nations, it will lead to changes to entire demographics of nations over 2 generations or so.