Vanguard’s total ownership of the U.S. stock market could hit 30 percent -- if it ever reached $20 trillion in assets, according to Bloomberg’s estimate.

Vanguard founder Jack Bogle, who died Jan. 16 at age 89, was concerned about the firm’s massive growth. He called the concentration of ownership a worrying trend, and wrote that a few passive asset managers could end up owning too much of the stock market.

Vanguard still holds only about 7 percent of U.S. equities and has a group dedicated to corporate governance.

So Vanguard has plenty of room to expand. Using Apple as a microcosm of the entire U.S. stock market, Vanguard funds collectively own 7.2 percent of the tech giant’s shares, but its biggest fund has only 2.1 percent -- well below the 10 percent limit.

Even if that threshold is reached, Vanguard could in theory simply launch a new version of the fund to get around the rule. If Vanguard and BlackRock sustain their growth rates, regulators might be pressed to step in and cap the amount that asset managers can own.

A four-fold expansion of Vanguard’s assets to about $20 trillion would give it control over about 30 percent of the U.S. stock market -- a level that could be reached in less than 20 years at the company’s current pace, Eric Balchunas wrote.

Vanguard doesn’t actually own the shares; rather, the investors in its funds do.

“It’s hard to see what would trip up Vanguard’s ability to grow assets," said DeMaso. However, Vanguard recently outsourced responsibility for voting in some actively managed funds to the portfolio managers, a decision Jeff DeMaso said “makes sense.” Those who pick the stock should be allowed to vote those shares.

Vanguard’s dominance across market environments and product types is increasing in 2019, indicating government intervention may be the only thing that could halt its growth, Bloomberg said.