[michaelt]

Sure, but if you retired in 1997 to live off the interest on an investment in the S&P 500, by 2008 you'd have made an annual return of... 0%.

If you used cromd's figures of $5M in funds and lifestyle expenses of $400k/year you'd have spent ~80% of the principal (although you'd probably have reigned in your spending unless you were mad)

A boat doesn't just need to survive the harbour's average water level - it needs to survive low tide and high tide.