Guess what? It is already happening.
Among richer, more secular countries, the birthrate is not at the rate of replacement. Maintaining a steady population requires a birthrate of 2.1 In Western Europe; the birthrate currently stands at 1.5, or 30 percent below replacement.
In 30 years there will be 70 to 80 million fewer Europeans than there are today. The current birthrate in Germany is 1.3. Italy and Spain are even lower, at 1.2. At that rate, the working-age population declines by 30 percent in 20 years, which has a huge impact on the economy.