Maybe this is the gist:
1) Stocks become overvalued and melt down periodically, like they did in 2000-2003.
2) A Mustachian has a 15-20 year accumulation phase, and a 30-40 year retirement phase.
3) A melt down would be more consequential to portfolio survival during retirement phase as opposed to the accumulation phase because (a) working income has been cut off, and (b) larger sums are usually at stake, and (c) the portfolio must recover while being drawn down.
4) More volatile index funds like QQQ usually suffer worse drawdowns than less volatile index funds like VTI.
1 & 2) Therefore, a melt down period is twice as likely to occur during retirement as during the accumulation phase, because the retirement is twice as long.
1 & 3 & 4) Portfolio survival strategies mostly need to focus on reducing volatility during the retirement phase.
Conclusion: Invest in more volatile index funds like QQQ during accumulation phase, when being hit by a melt down is both less likely and less consequential. Invest in less volatile funds like VTI during retirement phase, when being hit by a melt down would be both more likely and more consequential.
And of course if you could find
some sort of gauge that could show when stocks are at risk from being overvalued, that could be a helpful decision tool even in accumulation phase.