Let's say you do this exercise and arrive at a discounted cash flow model that accurately predicts a 5-10% YoY appreciation in value for the S&P index. How do

Let's say you do this exercise and arrive at a discounted cash flow model that accurately predicts a 5-10% YoY appreciation in value for the S&P index. How do you explain why this appreciation in value isn't arbitraged away, as suggested by GP? In his words, "if 9.7% long-term returns were guaranteed, wouldn't everyone just borrow 30-year loans on margin at 5% interest rate and invest it in S&P?" This would effectively drive up the current day price and eliminate any projected future growth in index price -- invalidating the hypothesis that the S&P price will continue growing in the future.