In announcing the news of their resignation, they said the company had "evolved and matured" since its founding. In addition, we plan to continue talking with Sundar regularly, especially on topics we're passionate about!
The change is effective immediately and Alphabet's share price has nudged up by less than one per cent in after-hours trading on the news.
By contrast, Cook, 59, still has as many as 1.8 million restricted stock units worth about US$500 million set to vest through August 2021, according to a recent filing.
But should they feel the need, they can override any decision Mr Pichai makes - with little more than a parental "because we said so". Promoting Pichai to lead Alphabet may address some of these concerns. It's a theme they apparently hope to continue, with the two saying in their blog post that they "believe it's time to assume the role of proud parents" (albeit parents with majority voting rights).
The company has also faced allegations of failing to adequately address sexual harassment in the workplace and of straying from the ideals espoused by the founders in the company's early code of conduct, which included the motto "don't be evil".
"Even the greats move on". It's just another sign of how the company is changing.
It remains to be seen how big of an impact this management shakeup will have on Alphabet's operations moving forward, but it's probable that Page and Brin have been ramping down their involvement with the company for some time. Some of our "Other Bets" have their own boards with independent members, and outside investors.
In a blog post, Page and Brin reflected on the dramatic evolution of Google since it was founded in 1998.
Google's longest serving CEO is still Eric Schmidt, the former executive brought into the role in 2001 as a so-called "adult supervisor" for Brin and Page. That includes drone company Wing and self-driving vehicle firm Waymo.
Mr Page grew up in MI, where his late father, Carl, was a computer scientist and pioneer in artificial intelligence and his mother taught computer programming.
Those investments have solidified Google's lead in the nascent technology.
The parent company was meant to make the tech giant's activities "cleaner and more accountable" as it expanded from internet search into other areas like self-driving cars.
"Google is the vast, vast majority of Alphabet in terms of revenues, profit and everything else, so why not put the guy doing a great job running all of that in charge of (the) whole company?" said Bob O'Donnell, chief analyst at Technalysis Research.