There no longer can be any doubt that the creation of the first index mutual fund was the most successful innovation—especially for investors—in modern financial history. The question we need to ask ourselves now is: What happens if it becomes too successful for its own good?

The First Index Investment Trust, which tracks the returns of the S&P 500 and is now known as the Vanguard 500 Index Fund, was founded on December 31, 1975. It was the first “product,” as it were, of a new mutual fund manager, The Vanguard Group, the...