The S&P 500 is the best reference index for US equities, but I'd be wary about using its history as a base case for retirement planning. The 60 years that its existed in its present form have seen the US economy grow consistently in a way that's almost unrivaled in human history. There are lots of good arguments for why the US will not grow as fast over the next 60 years, but for me the best one is just regression to the mean. Probably the US economy will grow at a rate closer to the lower rates common in our countries throughout human history, and over a long time frame the S&P 500 cannot grow much faster than the really economy.
Forecasting 7% long-term real returns from the current state of the S&P 500 ignores the current environment - the S&P 500 is currently priced very highly relative to historical prices.
There's an interesting bit of analysis that estimates an upper bound of 3.8% -- 3.95% real returns from US Equities. Worth a read.
> Valuations today are in the 97th percentile of all valuations in history and the 83rd percentile of valuations over the last twenty years (itself a period of very high valuations). Rather than assume that they will revert back to some past average, let’s start by granting the very bullish assumption that they will remain exactly where they are today forever.
Edit: the same blog that has a great post "The Single Greatest Predictor of Future Stock Market Returns" which goes beyond "mean reversion" of equity valuations, to show how you can make a better explanation (and better long term forecasts) by considering supply and demand dynamics as investors, on average, shift their allocations of investments between equities, bonds and cash.