You need an increase in worker population or an increase in productivity per worker to increase the output of your economy. If populations are stagnating and in

You need an increase in worker population or an increase in productivity per worker to increase the output of your economy. If populations are stagnating and innovation is low, that would cause less growth overall. Consider that many countries have a lopsided age distribution, such as Japan, China, and some European countries. Immigration into those countries may in fact be useful if it balances their population pyramids.