Currently, the cyclically-adjusted earnings yield on the S&P 500 is only 2.6 per cent.

This is at crash-warning levels, by historical standards. 

Should a prolonged bear market occur, equity valuations might collapse. The valuations of the mid-1960s did not return for 30 years. 

The big threat, then, is that, as John Maynard Keynes allegedly said, “the market can remain irrational longer than you can stay solvent.” In effect, there is a shortage of speculators able to prevent equities from remaining durably mispriced. 

Martin Wolf FT 11 Julu 2021

https://www.ft.com/content/e3a621d3-5cfc-4410-bd3c-0fde3535582b