The biggest problem with the 6% figure is that it ignores inflationary pressure. If you're stuck with extremely low risk investments (as you are once retired), you should really be looking at levels around 1 to 2% over inflation. At 2% over inflation (long term rate around 5%), withdrawing 6% annually (growing the withdrawal amount to keep up with inflation), you'll only last about 20 years, assuming no mishaps. Withdrawing 4%, the commonly recommended value, lasts more like 35 years; the average American retirement isn't that long, but it has a significant increase in expenses at the end.
"you'll only last about 20 years" 20 years is long enough to take on a fair amount of diversified risk, which up's your return, which let's you take on more risk etc.
Also a 60 - 40 stock/bond is actually safer than a pure bond portfolio because of inflationary risks and has a higher expected return over 10+ years. The only real trick is to avoid single investments, be they a stock or an industry.
