It is based on an interpretation of economic and financial history that is just flat wrong, and actually and deliberately deceiving - deliberately to sell stock mutual funds and other investments to the public.
If, for example, you look at the level of stock prices on the New York Stock Exchange (NYSE) from 1850 till 1945 and somehow track down all the dividends and market values year to year you could construct a rate of return on "equity" on the NYSE. The problem is what you have constructed does not represent the rate of return on equity in society. What you have constructed indicates the rate of return on stocks listed on the NYSE. An investment in American business equities in 1850, 1860, 1870, 1880, 1890, 1900, 1910, 1920... would have entailed buying shares the myriad farms, general stores, and other businesses that dominated the economy but which did not trade on the NYSE. A few insurance companies and railroads that did trade on the NYSE did not represent the American economy in 1850 or in the years after, and the industrial companies that cropped up around 1900 do not represent all of it either. The share of income going to farming and the businesses that were not listed on the NYSE declined steadily from 1850 till 1945, which an absolute fact readily available in U.S. Economic Census reports. The reason for that is those areas of business activity returned far less, or in the case of farming effectively nothing above the cost of labor, compared to the equities that happened to be listed on the NYSE. By only looking at the equity traded on the NYSE, year to year, you cherry pick, by design, the winners and ignore the losers. If one had actually invested in equity across the economy over time one would have found that the return was far lower than the return from the stocks on the NYSE or any other stock market in the past.
If you fast forward from 1945 to today you will seem a similar thing going on. As you get closer and closer to the present you will find that the stock market much more closely matches the equity available to invest in America than it did in the past. However, since the stock market returns from investing in equity in the past do no represent the return from investing in American "equities" or world "equities" the stock market's past performance does not indicate its future return. The correct approach to predicting a rational return from stocks would be to figure out what the actual rate of return from investing in American business equity or international equity actually was in the past. If one does that they will be very disappointed. Mathematically it cannot be more than the rate of increase in GDP plus whatever share can be pried away from labor income and interest income... With debt spiraling up with no end in sight, the interest share will only rise squeezing out the share that could go to stocks. As labor's share declines politicians like Obama get elected on a mandate to redistribute shareholder's income to others...
The stock market's return as pathetic as it is in the past 15 years is actually quite typical of what one could have expected from investing in businesses on a diversified basis in the past. Only the period 1945 to 1980 shows strong returns - that is a product of rebuilding the world economy after World War II and much higher returns on capital until was rebuilt.