Even if
we observe rich valuations, there can be some justification for accepting market risk during periods when market internals are uniformly strong, provided that the environment is not also characterized by a syndrome of overbought, overbullish and rising - interest rate conditions.
It's important to distinguish between the level of
valuations, which has indeed become breathtakingly extreme in recent years, and the mapping between
valuations and longer - term market returns (which we
observe as a correspondence, where
rich valuations are followed by poor returns and depressed
valuations are followed by elevated returns).