The Tim Cook era is coming to a close with an existential challenge for Apple: figuring out what comes after the iPhone. Why it matters: Cook extended the iPhone's success into products like the Apple Watch and AirPods and built a powerful services business. But the company hasn't broken into a major new category — and has stumbled into the AI era. Driving the news: Apple announced Monday that Cook will cede the CEO reins to hardware chief John Ternus, while remaining with the company as executive chairman. The company also promoted Johny Srouji, the driving force behind Apple's chip success, to the new role of chief hardware officer. That could entice Srouji, whose name is often mentioned in tech CEO searches, to stay in Cupertino. The big picture: Cook has executed masterfully to maximize the company's iPhone success and transform Apple into one of the world's most valuable companies. Yes, but: His efforts to expand far beyond that device have largely sputtered.…