The arguments of Buffett and Bogle go something like this (I'm being a little loose with the numbers) ...
The total equity return in the 20th century was 10%. 5% of this return was due to market-value gain (e.g., Dow increased from 66 to 11,497) and 5% was due to dividends. Today, the dividend rate is only 2%. Hence, if we assume a 5% market-value gain and a 2% dividend rate for the 21st century....
I believe this argument is flawed because it doesn't consider the cause of the lower dividend rate. Why is it only 2%?
A lower dividend rate can be caused by the over valuation of equities. In essence, if equity prices are much higher than their earnings (PE ratio) then the dividend rate will be low. However, the PE ratio today is only slightly higher than the average PE ratio during the 20th century, which was about 15. Stocks are not excessively overvalued today and this is not the primary cause of the 2% dividend rate.
[deleted lots of good analysis about dividend rates]
For these reasons, I believe that the lower market return estimates of Buffett and Bogle are flawed. Granted, they are incredibly wise and experienced individuals with demonstrated excellence. However, I've never seen them address the cause and implications of the lower dividend rate.