Is Google's Page dealing with thinning of ranks?

Google cofounder and Alphabet CEO Larry Page speaks during the Google I/O Annual Developers Conference in San Francisco.
David Paul Morris | Bloomberg | Getty Images

First it was Marissa Mayer. Then came Vic Gundotra, Nikesh Arora and Andy Rubin. Now it's Alan Eustace. While Google is a massive company loaded with tons of smart people, those are some high-profile departures for Chief Executive Officer Larry Page to digest.

Whether executives left because of internal struggles (Gundotra, Rubin), to run other companies (Mayer, Arora) or, like Eustace, to do whatever they want, all these people were in charge of big initiatives at the Mountain View, California-based company.

The fact that some of these projects (Google Plus), haven't been as successful as others (Android), doesn't change the reality that such talent will be difficult to replace. And the red-hot start-up market will likely only make it harder for Google to attract the risk takers that mature tech companies need to maintain a high level of innovation.

It's hard to blame Eustace, who was the company's senior vice president of knowledge (a title only Google could ascribe), for wanting a change of scenery. He spent 13 years at Google, joining two years before the initial public offering, and worked on key products like search and maps. Eustace has plenty of extracurricular interests to fall back on—he recently set the world record for the longest free-fall jump at almost 26 miles.

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It's clearly Page's Google now, with co-founder Sergey Brin riding shotgun. Page is responsible for charting the company's direction, be it the $3.2 billion purchase of smart thermostat maker Nest or the acquisition and then divestiture of Motorola.

He's tasked with making sure the Android and Chrome teams get along, that shareholders are OK with far-out investments like self-driving cars and a network of inflated balloons, and that the billions of dollars the company generates quarterly from search are wisely deployed toward mobile and the platforms of the future.

"Larry is very much putting his own stamp on the company," said RBC Capital Markets analyst Mark Mahaney, who has a "buy" rating on the stock. "The company had been run by a loose triumvirate. One thing Larry has done is try to simplify the organization, get rid of silos and move on beyond inefficient managers."

Even before the Eustace announcement, Argus research analyst Joseph Bonner cited "the retention of key personnel in the highly competitive internet technology sector" as one of Google's key risks. Bonner, who has a "buy" rating on the stock, named Mayer and Arora, among other top execs to leave.

And there have been plenty of departures. Mayer, one of Google's first employees and central to the simple user interface, left in 2012 for the CEO job at Yahoo. Gundotra departed in 2014 after struggling to make Google Plus relevant, and Arora, a decadelong employee, followed soon after to become vice chairman of Japanese Internet giant SoftBank. Rubin, who created Android, announced his exit in October to build a start-up incubator.

Read MoreIn defense of Marissa Mayer

For Google, these sorts of departures are the price of success. The company has made its early employees fabulously rich and opened countless doors for those managing products and big teams.

"You've got a big company that has created a tremendous amount of wealth for a very concentrated number of people," said David Steinberg, CEO of Zeta Interactive, a New York-based digital marketing company he co-founded with former Apple CEO John Sculley. "People that come in and out of the ecosystem are very focused on doing creative and interesting things, and the bigger a company like Google gets the harder that is to do."

A Google spokesman declined to comment for this story, beyond confirming that Eustace is retiring and staying through March "to help with the transition."

Concerns over Google's discipline
Concerns over Google's discipline

Page, who co-founded Google in 1998 and owns about $30 billion in company stock, is as invested as anyone in avoiding a brain drain. Given the types of projects he's funding, one would seem unlikely.

Since he became CEO, Google has jumped into virtual and augmented reality, started testing drones, built software to power smartwatches and connected cars and opened an investing unit to back late-stage tech start-ups.

The company's Android operating system has also taken over the lead position in global market share, and Google controls 35 percent of the $29 billion U.S. mobile ad market, according to eMarketer's 2015 estimates.

Read MoreGoogle aims to dominate Facebook in virtual reality

All the while, Google's cash stockpile has almost doubled since the end of 2010 to more than $64 billion, thanks to increased dominance in Web search. And though the stock has had a rough 12 months, dropping 7.4 percent, it's still up 73 percent on Page's watch, topping the broader U.S. market.

The way Zeta's Steinberg sees it, Page has been so successful that anytime one of his moonshots, like Google Glass, has a slip-up, the public jumps all over him. Google said in January that it's temporarily ending sales of the computerized eyewear as it works to move the product "even more from concept to reality."

"It speaks to what a good job they're doing that when they have a failure that small it becomes front-page news," Steinberg said.

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But there's one thing Page can't fix: Slowing growth in Internet search. For more than 16 years, Google has been milking that business for all it's worth. But consumer activity is shifting to mobile devices, and quickly. For the fourth quarter, Google reported revenue growth of 15 percent, down from 22 percent two years earlier (excluding Motorola).

Google is pouring resources into mobile apps and reimagining search for smaller screens and consumers on the move. On the latest earnings call, Chief Business Officer Omid Kordestani said "mobile is now a behavior, not a device."

Can Page hire and retain the smartest minds in mobile advertising to track that behavior?

It's not as sexy as driverless cars, but it's where Page needs to win so he can keep making the crazy bets that land on the front page.