P/E mean reversion can occur via two routes - a decrease in P or an increase in E. Whose to say we will not have the latter? Global growth is happening right now.
You also need to look at equity valuations against other asset classes. If you want to sit in cash during a period of financial repression be my guest. Or gilts, gold or property after their respective bull markets and now derisory yields? Ultimately money has to go somewhere so it becomes a relative game.
Also worth bearing in mind that whilst equities may still be above their long-term valuation average that average has produced real growth of some 5% pa long-term. Massively outperforming all other assets.
I agree that one should buy shares (not sure how good the average PI is at spotting quality!) at sensible prices but I differ in that I would then say one should hold them for the long term rather than trying to box clever with timing. Unless or until the next obvious bubble comes along and we are nowhere near that currently.