What these things fail to take into account is the growth in the amount of money going into the stock market.

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.