There was certainly a negative return over 20-year periods including the Great Depression.
And negative real returns during the 1970s stagflation.
Ask Germans or Russians what the worst rate of return for a 20-year period was in their country.
I'm not sure what the return on all US stocks is since then, but Japan's market has only just returned to the level it was at in its 80s bubble.
History
In 1860, Henry Varnum Poor formed Poor's Publishing, which published an investor's guide to the railroad industry.[20]
In 1923, Standard Statistics Company (founded in 1906 as the Standard Statistics Bureau) began rating mortgage bonds[20] and developed its first stock market index consisting of the stocks of 233 U.S. companies, computed weekly.[1]
In 1926, it developed a 90-stock index, computed daily.[1]
In 1941, Poor's Publishing merged with Standard Statistics Company to form Standard & Poor's.[20][21]
On March 4, 1957, the index was expanded to its current 500 companies and was renamed the S&P 500 Stock Composite Index.[1]
As you allude, there were 20-year periods spanning the 1970s where Treasuries outperformed large-caps, but that’s fairly rare (and with the printing presses running three shifts, something we’re in extremely little danger of right now).