You can see the same adivce everywhere:
"Just buy S&P 500 and hold, don't think too much".
Yes, it works most of the time, but does it still work when valuations are this high?
You can see a lot of examples where buying at extremes led to 10 or 20 years of flat returns. The Nikkei is the classic one, but the S&P has had long dead periods too if you bought at the wrong time.
Valuations are really high right now, a few mega caps carrying most of the index and massive passive inflows pushing prices up. I'm not saying to go all cash or try to time the market, but at what point does price actually matter?
A few questions: do you change anything when the market looks to expensive? Do you focus more on other markets, or just accept lower returns?