Apple announced that Tim Cook will step down as CEO in September after 15 years in the role, handing the job to hardware chief John Ternus. Longtime Slashdot reader sinij shares the news from MarketWatch: Cook leaves an impressive legacy after growing the company to a $4 trillion market capitalization from just $300 billion 15 years ago. Over Cook's 15-year tenure as CEO, Apple's stock has risen 1,932%, beating the S&P 500's 504% increase, according to Dow Jones Market Data. That places Apple's stock as the 38th best-performing member of the index over that period of time. Cook had big shoes to fill, replacing Apple's iconic founder, Steve Jobs, as CEO. Cook's successor, John Ternus, Apple's senior vice president of hardware engineering, will need to guide Apple's through uncharted waters as the company navigates its artificial-intelligence transition and supply-chain constraints. Cook will remain at Apple as executive chairman.…