When the market is just going up, up, and up, we all tend to be blind to the holes in the market. They're all papered over by the rise.
The mutual fund industry has been very successful at promoting itself in this glamorous way. Mutual funds are actually vast, impersonal bureaucratic structures, and yet they were clever enough to give a human face to the mutual funds.
We really haven't had very much experience with people funding their retirement out of the stock market, and we don't know, frankly, how it would work under every scenario.