Tim Cook is stepping down as CEO of Apple. In nearly 15 years as chief executive, Cook turned Apple into a global powerhouse, building on the legacy of his legendary predecessor Steve Jobs. Why it matters: Cook, who oversaw the launch of the Apple Watch and AirPods , will be replaced by hardware expert John Ternus . The big picture: Apple is one of the most valuable companies in the world. Driving the news: Cook will become executive chairman of Apple on Sept. 1, the company said Monday. Ternus, senior VP of hardware engineering, has been widely viewed as Cook's likely successor. What they're saying: "It has been the greatest privilege of my life to be the CEO of Apple and to have been trusted to lead such an extraordinary company," Cook said in a statement. By the numbers: Apple's market value grew from about $350 billion in 2011 to roughly $4 trillion under Cook's tenure. Annual revenue nearly quadrupled to more than $400 billion. Active devices topped 2.5 billion worldwide.…