Tim Cook has transformed Apple Inc. since taking over as chief executive officer from Steve Jobs in 2011, introducing new product categories (the smartwatch), pushing into new businesses (streaming video), and making an audacious attempt to take a new type of computing (mixed reality) mainstream. He’s served for far longer than the average Fortune 500 CEO, and, at 63, is older than many of his peers. But if it seems like a logical time for Cook to start planning for someone else to shape Apple’s...