How much are long-term returns based on population growth?

People regard historical market returns as almost a law of nature for the future.

However, the US is starting to do something it has never done before, which is stabilize in population. And the population growth rate probably won't return to what it always was, ever, within your lifetime or mine.

How much of the past economic growth was not due to new technology, or improved policymaking, but just a population pyramid scheme?