Sure, that's nice. (Although it seems to have been heading somewhat lower for the past decade.) But if you'd started investing in 1998, say, and then in 2008 something happened such that you unexpectedly needed access to your 10 years' worth of accumulated resources, things wouldn't have looked so rosy.

Overall, the long-term trend may be pretty reliably positive. But that's little comfort to an individual who may not have the luxury of waiting out a poorly-timed, lengthy downturn.