Also, GMO (the asset manager he founded) publishes 7-year asset class return projections. Because they are based on a reversion to mean valuations at that horizon, and valuations are very high now, those projections are markedly negative for US stocks. In the interview below, he offers a 20-year projection. (The effect of falling valuation is less on a longer horizon.)
Here is the interview: https://www.wsj.com/articles/jeremy-grantham-predicted-two-previous-bubbles-and-now-1509937980
His 20 year projection for total, real US stock returns is 2.8% per year.
He expects valuations to fall 2/3 of the way from current levels toward the historical long-term mean, to reach a new, higher long-term mean. The current PE10 is about 31.3 and the long-term mean is about 16.8, so Grantham's estimate of the new "fair value" is about 21.6.
Note the current forward dividend is about 1.94% and historical real earnings growth (from Shiller, 1881 to present) is about 1.8%, so the future return can be estimated at 3.74% with constant valuations. Grantham expects a roughly 1% haircut due to falling valuations over the next 20 years.
In my opinion, that is reasonable, but to the extent it is incorrect, I think it is a bit more likely to be lower than higher.
If you are basing your plans on a historical 6.8% total real return to stocks, or even a "conservative" 4%, then you are likely to be disappointed, in my opinion.
***
I once made a 10-year bet with a colleague (payoff was supposed to be a beer), so I'll offer a 20-year bet here: The 20-year total real return of the S&P 500 from today will be less than 4%. The S&P 500 is at 2589.96 as I write.
I don't really care for beer, but if anyone would like to suggest a payoff of similar value I'll go for it.*

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