I ran across Bogle's name soon after I entered the business news world more than 15 years ago. By then, he had spent decades carving out a reputation as an outspoken foe of high fees, revenue-sharing deals and anything else that might erode people's mutual-fund account balances.

Bogle more recently caught my attention when he lent his support to the Department of Labor's drive to expand the fiduciary standard.

"I've been speaking about a broad standard of fiduciary duty for as long as I can remember," he told CNBC. "I hope that one day we get that standard of fiduciary duty that applies to everyone who puts their hands on anybody else's money. This is a good start." 

Also read: Obama backs tougher fiduciary rules

Though long ago retired as Vanguard CEO, Bogle was and has continued to be that rare breed of thorn-in-the-industry's side and legendary business insider at the same time – and never mind the fact that he's suffered at least six heart attacks and had one heart transplant.

His philosophy, of course, is that only fools try to beat the market and that you're throwing good money after bad if you've turned to fund managers.

It's an approach that has worked very nicely, thank you. Vanguard today manages more than $3 trillion and is the second-largest money manager in the world, after BlackRock.

But Bogle is 85 now and, though he apparently still comes to work every day, I can't help but wonder who might step in once he decides he's had enough.

It's a question plenty of people are thinking about, no doubt, because Bloomberg News just published a list of who it thinks might step into Bogle's shoes.

Among the people on its list were Bill McNabb, the CEO of Vanguard; Mark Wiedman, global head of Blackrock iShares; and Bogle's son, John Bogle Jr., founder of Bogle Investment Management.

Wall Street has more than its fair share of strong advocates. The mom-and-pop investors who it's supposed to serve need a figure equal to Bogle in the years ahead.

Last month, the Financial Times ran an article about the speculation swirling around legendary investor Warren Buffett's succession plans as CEO of Berkshire Hathaway.

"No one who steps into his shoes will have Mr. Buffett's authority or credibility," Buffett expert Larry Cunningham, a law professor at George Washington University, told the newspaper.

While none of the people on Bloomberg's list is a slouch, I'm afraid that much of the same can be said about John Bogle's replacement — should they ever be found.

Also read: Why John Bogle's Senate testimony rocks