There are many, many 20 and 30 and 40 year and even 50 and 60 year periods where the market experienced losses and some cases ghastly losses. One could not invest in the "DJIA" back then because there were not etfs, nor were there retirement plans, lifespans were shorter (so the "40 year" arbitrary benchmark applies somewhat less) and the DJIA also has "survivor bias", which means (as you surely know) that your investment in Eastman Kodak or Bethlehem Steel (once the largest company on the NYSE), or even GM for that matter which go to zero are replaced by something else.