Provided the overall market and economy continue to grow in spite of short term volatility, it would happen again. If one honestly believes that the whole market is going to go down over time, then probably best to look at bonds or precious metals.
Yeah, the condition you mention is the key: what are the odds that the US economy (or the overall world average, if you're diversified with a global index) will continue to grow as gloriously as it has done in the past 50 years? (when the guy started investing)