Stocks are worth exactly what people are willing to pay for them - no more and no less. Thus, a value judgement like "overvalued" or "undervalued" is meaningless. For what it's worth, the current bull run started in the early 1980's with the Dow average at 880. Could we retrace the last 20 years? Sure.
Will we? I don't think so, but many would certainly beg to differ.