A red flag for investors could be if McNabb tried to steer Vanguard away from its commitment to remain a shareholder-owned business.
"In general, Vanguard remains unwavering in its commitment to the mutual ownership structure. That's at the core of what Vanguard's all about," Culloton said. "If that ever changes, then we'd be facing a real change in Vanguard's culture."
But he doesn't see that happening under McNabb. Neither does David Fernandez, a former Vanguard investment manager who now runs Wealth Engineering LLC, a Scottsdale, Ariz.-based financial consultancy and advisory service.
Still, he was shocked by Friday's announcement. Fernandez says that Brennan was known for his energetic demeanor and good health.
"So this is very surprising," he said. "Brennan has built Vanguard into a global financial conglomerate with a leading ETF position. It's surprising that he'd pick this time to leave considering where the business is at now and his relatively young age."
The timing of his departure leaves the door open for speculation, Fernandez says, "that something else is going on behind the scenes."
But McNabb, he added, "is someone who's well-known within Vanguard and highly regarded. The departure of Brennan shouldn't change Vanguard's investment focus or the appeal of its products to shareholders."