Who says you have to invest in Dow Jones' market during the 21st century? I'm sure it wouldn't be hard to get a 5.3% return in a more emerging market like, say, India or China.
Even if it doesn't, though, everyone knows that the trick to successful investment is to buy low and sell high. There are still plenty of opportunities to buy low - even with limited risk - in large emerging economies. The U.S. may become a low-risk capital preservation stock and China will become the higher-risk investment stock.
1) Emerging markets tend to be riskier, so if you look at it on a risk-adjusted basis the expected returns don't look as good. What happens to your assets if China has another revolution?
2) As a practical matter, an individual US citizen just can't buy equity in many emerging market companies. Either they aren't listed on US exchanges, or are privately held, or national goverments have ownership restrictions in place. There isn't even much in the way of mutual funds to do it indirectly.
3) The few stocks in the BRIC markets available for direct purchase by individual outside investors have already been bid up to ridiculous levels by dumb money who think they can't afford to be left behind. You may be waiting a very long time to see much significant upside.