If you are an investor in Apple, it's hard to argue with his performance over the years Tim has been at the helm. Apple is one of the most valuable companies in history (arguably the most valuable), it has successfully diversified its income stream, its stock has done well overall, and its products have sold well and commanded high prices. Apple also successfully navigates one of the most complex supply chains ever. All of those are measures of success.
It's not even quite fair to assert that he's not a product visionary, since he introduced the iPad mini and entirely new -- and arguably groundbreaking -- products in the Apple Watch and AirPods, to name just two. I happen to love both of those. Apple's microchip development is also nothing short of astonishing compared to what most analysts thought was possible.
That said, I agree with others that there's something missing. The Mac, though still updated, has seemed like an afterthought since Tim has been here. He seems to completely misunderstand what many people want from that platform, and he has never unleashed the Mac team to be everything it no doubt wants to be. While Apple has always jettisoned legacy technologies, Tim has eliminated from Apple's various products even things necessary to many users, such as headphone ports and MagSafe. Product matrices are confused (see the MacBook line), and Apple has fallen behind on touch-capable PCs (make no mistake, Steve Jobs would have figured out how to add what works without undercutting his points on what doesn't work with touch). The current MacBook keyboard situation would have been intolerable to Jobs and to most tech CEOs. And Tim doesn't seem to have a clear plan on where iPhone should go from here.
In short, I generally think Tim has done a good job. But I also think it's about time for a change.