Whenever an existing company raises funds by selling new stock, the total value of the stock market rises. Meanwhile, the value of the index fund portfolio stays put. As a result, the growth rate of the total stock market regularly outpaces the capital gains growth rate of an index fund portfolio. The empirical factors all point in the same direction. Capital gains growth rates aren't likely to be as strong in the future as they were in the past.
Dividend yield refers to the value of the dividend paid, divided by the value of the stock.
To estimate future dividend yields, one must be able to estimate future movements in both the numerator and the denominator. Stockholders don't approve of dividend payments that bounce around from one quarter to the next, and corporations are loath to anger their shareholders. Moreover, if corporate dividend payouts are measured in GDP terms, the same stability can be observed. As a result, it is not so difficult to forecast future trends in the dividend yield numerator. Now, however, the nature of the game has changed considerably.