I disagree. Tim Cook has been pretty solid. He's no Steve Jobs, but he's done a fine job. Although he hasn't produced any new cash cows so far, he's been a decent steward of the ones they already had and he's taken the company into new markets with the Apple Watch. It's not the most inspiring product, but it is the premium wearable and clear leader. Then there are the AirPods, which fit in nicely in wearable category. And I love what Apple is doing with their own silicon.
Sure, there are no home runs here, but he's effectively executing a coherent strategy and hasn't really had any big mistakes. I wouldn't put him as high up as Nadella and I certainly wouldn't put him as low as Ballmer. It's tough to be the guy who follows the founder and Cook has handled it well. I don't know who is out there who looks like a better candidate or a suitable replacement for Cook.