The truth is, the existing proposal doesn’t go nearly far enough. It is limited to retirement plan accounts and ignores the other three-quarters of the assets owned by individual investors. Any effective rule must encompass all investors.

It is widely agreed that the fiduciary rule would give impetus to the growing use of lower-cost, broadly diversified index funds (pioneered by Vanguard, the company I founded), such as those tracking, with remarkable precision, the S&P 500 stock index. But even without the rule, there has already been a tidal shift to index funds — actually, more like a tsunami. Since 2008, mutual fund investors have liquidated more than $800 billion of their holdings in actively managed equity mutual funds and purchased about $1.8 trillion of equity index funds. Low-cost index funds are almost certainly what Mr. Cohn means when he refers to the “healthy food on the menu.”

Several major brokerage firms have already embraced the fiduciary principle, announcing plans to comply with the rule by eliminating front-end commissions (known as loads) on retirement plan accounts in favor of an annual asset charge. And dozens of companies have reacted to the proposed rule by creating a class of generally less costly mutual fund shares with initial loads of 2.5 percent followed by annual charges of 0.25 percent of total assets.

I do not envision these responses to the fiduciary rule being reversed. With or without regulation by the federal government, the principle of “clients first” is here to stay.

In the debate about the fiduciary rule, one basic fact has been largely ignored. Investment wealth is created by our public corporations and reflected in stock prices. Stock market returns are then allocated between the financial industry (Wall Street) and shareholders (Main Street). So when the consulting firm A. T. Kearney projected that the fiduciary rule would result in as much as $20 billion in lost revenue for the industry by 2020, it meant that net investment returns for investors would increase by $20 billion.