That’s true. But the investor will also NOT be selling at book value. A business that’s able to consistently generate a high ROE will always command a high valuation
Unless the business was growing at a high rate before you purchased the stock, and the growth later stops, I don’t see why the stock would command a lower PE. Since the statement by Munger was about ROEs, I think it is fair to assume the condition of “Ceteris Paribus” (ie.) assuming growth to remain the same throughout the 20 year period.