What type of returns can investors expect over the next decade?

Last month, legendary investor Stanley Druckenmiller said there was a “high probability” that the market will be flat for a decade. Is he spitting out facts, or is he Drucken outta his mind?
He argues that the factors that created the bull market since 1982 are no longer there:
If you bought the S&P 500 at the peak, go back to bed. It’s going to be a snooze for a couple of years.
Say the S&P 500 grows at the historical ~9% average (incl. dividends) — it would take just under three years from yesterday’s price to break new highs.
That’s also assuming two things:
But the recovery time will vary depending on at which point you bought in the cycle. If the annual compounded return drops to 5%, we’re looking at nearly five years to return to break new highs.
Those that joined the market in the past decade are used to double-digit yearly growth — thanks to low interest rates and easy monetary policies (Fed printing money).
Most economists and analysts have been very wrong with forecasts this year. It’s almost pointless to focus on short-term price predictions.
The Average Joe: “It’s a good thing I’m farsighted. Can’t see sh*t in the short-run — but the distant future is crystal clear.”