They can't scale their "magic".

Working on the watch means that they aren't working on the Mac or the iPad. They are a victim of their own success in that a successful, well thought out, and profitable product like the watch is viewed as a neutral.

They've also flopped on the TV.

That said, their ability to execute is amazing.

I'd say that "running out of time" is probably over the top, but there's some truth behind the idea. Their huge margins and huger stock price and market cap are based on the perception that they're the company that can produce industry-changing smash hits over and over again. But Jobs was really the one doing that. Can Cook's Apple produce the next big thing, something that takes the whole tech world by storm, like the iPhone or iPad did? How long will investors wait if he doesn't make any big hits? It's not hard to imagine Apple losing some of it's luster if they keep churning out modest revisions on existing products and uninspiring duds for new products.

On the other hand, even with no more hits, they do still have incredible customer loyalty and a massively profitable product line. And a lot of people have predicted Apple's doom over the years, only to be proven wrong later. Betting on Apple for the next 5 years seems safe. I wouldn't bet on Apple for the next 20 years right now though.