The only debate is about how fast and how far the eventual fall will be.  For the first time ever, the ratio of U.S. stock prices to U.S. GDP has reached 200 percent.  In other words, the total value of U.S. stocks is now twice as high as the value of all U.S. economic output for an entire year.  To get an idea of how crazy this is, just check out this chart.  Historically, the ratio of U.S. stock prices to U.S. GDP is normally under 100 percent, and so if all stock prices were cut in half U.S. stocks would still be overvalued.  That is how extreme this bubble has become.

Other key valuation measures also indicate that stock prices have gotten wildly out of balance.  The following example comes from a Motley Fool article entitled "Here's Why You Should Expect a 20% Stock Market Crash in 2021"…

Looking back 150 years, the S&P 500 has averaged a Shiller P/E of 16.78. Admittedly, the Shiller P/E ratio has been a lot higher over the past 25 years. The advent of the internet has broken down information barriers for retail investors, and historically low lending rates for more than a decade have fueled borrowing and lit a fire under growth stocks.