Also because so many companies are staying private, a crash in private markets is relatively irrelevant for the overall economy.
Simplifying an economic activity down to a single short formula leaves out a lot of important parameters and these kinds of things tend to hold some truth for the time they are invented and often break at some point in the near future after they are created. This is because of changes in money flows in the economy as a result of tax, regulation and technology changes.
Like the yield curve inversions and the Sahm rule and so on.