So the decline of population growth from 2.2% per year in the 1960s peak to 1% now and 0% projected 60 years from now means exactly the same decline of return of investments. I can't see it as a big problem at all. Economy is growing not because of population growth but because of technological development (and until the 1970s, also because of ever expanding energy extraction, but that's over a long time ago).
So the decline of population growth from 2.2% per year in the 1960s peak to 1% now and 0% projected 60 years from now means exactly the same decline of return of investments. I can't see it as a big problem at all. Economy is growing not because of population growth but because of technological development (and until the 1970s, also because of ever expanding energy extraction, but that's over a long time ago).