Here’s a passage I found worthy of consideration – from Vaclav Smil’s Global Catastrophes and Trends – The Next Fifty Years (2008):
The aging of the U.S. population, although far less pronounced than in Europe or Japan, and a multitude of social ills will only accelerate the inevitable transformation of the country. The aging of the population will have similar effects on health budgets, pensions, and the labor market as in Europe or Japan, but given much more widespread stock ownership, its principal undesirable effect may be on the value of long-term investments. There will be too few well-off people in the considerably smaller post-boomer generations to buy the stocks (and real estate) of the aging baby boomers at levels anywhere near peak valuations. That is why Siegel (2006) expects that stock prices in rich countries could fall by up to 50% during the coming decades, unless newly rich investors from Asia, the Middle East, and Latin America step in. But that intervention would have to be on a truly massive scale. Siegels calculations indicate that for rich countries’ stocks to perform at their long-run historic rate, most multinational corporations would have to be owned by non-Western investors by 2050. (p. 151)
While I haven’t studied the details, it strikes me that there has to be some truth to this story. The reference is to the book The Future For Investors by Jeremy Siegel.