>It's been a really bad idea for many long historical periods.
Name a historical period of longer than 15 years in which the market as a whole has lost money relative to inflation. That didn't even happen across the Great Depression.
Furthermore, market history did not begin in 1910 in the United States. Stock markets go back hundreds of years in many countries. A global long-view perspective is worth a lot more than 90 years of US data. And from that perspective "stocks for the long haul" have very often been a bad idea.
Your chart shows a flat spot of 16 years across the 80s, and another one of about 25 years measured from the very peak of the market in 29.. So you're right, though that figure isn't impressively different.
The figure I was remembering was actually about a 60/40 balanced stock+bonds fund like VBINX, which is what I'd recommend to anyone anyway.
Name a historical period of longer than 15 years in which the market as a whole has lost money relative to inflation. That didn't even happen across the Great Depression.