The first chart plots forward 10-year returns for the S&P 500 at various starting 5 point "CAPE" valuation buckets (i.e. less than 10x P/E all the way through above 30x) against the change in the starting P/E relative to the P/E in ten years (i.e. whether the P/E multiple expanded or contracted) going back to Ibbotson data inception in 1926. The chart shows the strong relationship between forward performance and the change in the multiple, as well as the impact of the starting valuation (the cheaper the starting valuation, the higher the returns and the more likely the index will exhibit multiple expansion, whereas the more expensive the starting valuation, the lower the returns and the more likely the index will exhibit multiple contraction).
The next chart plots the same information, but uses forward real returns (i.e. adjusted for inflation). It is interesting to see the tight convergence of returns during periods of P/E multiple contraction irrespective of starting valuation, indicating that some of the decent nominal returns during contractionary periods in the first chart at lower starting valuations occurred during inflationary environments (mainly the 1970's).
So where do we currently sit... at the current 28.3x CAPE, decent forward returns will require the multiple remaining elevated (or becoming more elevated) as no change would equate to a roughly ~4% real return in the model. While no change is certainly a possibility, the below chart shows the CAPE has declined in all previous 67 ten year periods since 1926 when the CAPE was greater than 28x, with an average and median decline of around 40% (which would take us right back to the historical average of ~18x), which at the current valuation models out to a roughly 0% real return over 10 years.
None of this is a sure thing, especially over the short-run. Despite being expensive three years ago, the S&P 500 has returned 10% annualized since. It just happened to have benefited from moving from the 15th percentile of most expensive CAPE to the 6th most expensive. While it absolutely can get more expensive from here, that's simply not the long-term buy and hold bet I would want to make when there are cheaper opportunities available outside and within the U.S..