The math in the statement is correct in the short term. If the risk-free rate is around 3% and the equity risk premium is about 4%, the expected return on stocks would be roughly 7% a year, which would double values in about 10–11 years using the Rule of 72. That doubling could occur for a time even without productivity growth, as companies can boost earnings through pricing power, cost-cutting, or financial engineering.

Over the long run, though, sustained returns depend on fundamentals like productivity growth, population growth, and inflation. Without productivity gains, corporate profits would eventually stagnate, making it difficult to maintain a 7% annual return. Risk premiums, interest rates, and valuations also change over time, so fixed assumptions rarely hold for decades. In short, the doubling math works, but it oversimplifies the economic reality that long-term stock growth ultimately relies on productivity.

My point was that is that even without any economic growth stock indexes grow quickly because they are cumulative (that is include profits of companies by either dividend paid or stock buybacks). Some indexes are not fully cumulative but they are still close enough because companies often prefer buybacks to dividends these days (for good reasons).