Apple stock has had a colossal run with Tim Cook at the helm. The Apple CEO is stepping down , set to be replaced by the company's hardware chief, John Ternus. Cook will assume the position of executive chairman. Shares dipped slightly on the news, with the stock down about 1% after hours. Looking back at the last 15 years of Cook's tenure, it's understandable that investors might be nervous about who will take over as the iPhone maker's top exec. The stock is up 1,946% since Cook took the