They've been incredibly profitable under his leadership, why would they fire a successful CEO who has increased profits, marketshare?
I may not be a fan of him, but using the metrics that business people use to measure success/failure. He's been an unabashed success.
He essentially has solved the "big company" problem for Apple, which in a nutshell is that large companies can't achieve the same growth as smaller companies, all else being equal. Thus, and especially in the tech sector, companies that grow very quickly as Apple did often plummet after that. So Cook should get credit for restructuring Apple and its finances to sustain the business legacy that Jobs left. He also brings tremendous supply-chain expertise (a huge thing for Apple, which routinely must acquire components in insanely high numbers, from all corners of the world, in near secrecy and extremely quickly) and has pushed into emerging and difficult markets very successfully. And it's not insignificant that Apple under his leadership has championed data secrecy and human rights in ways that have made a difference. All of this supports your point that Cook has been successful in many ways related to Apple's business.
But is that enough? I would argue that for Apple, it is not. First and most importantly, Apple's legacy is one of innovation, technically and culturally. Although Cook's Apple has innovated to a certain extent with the Apple Watch and the AirPods, none of this is "disruptive" in the way that Jobs achieved multiple times. Worse, Apple is squandering obvious advantages nearly everywhere one looks. Given how many people viewed the MacBook Air as a near-perfect consumer laptop, it's rather astonishing how Apple failed to realize many of that product's strengths when it created the MacBook. The same is true of the MacBook Pro line: the new computers are quite nice in many ways but they simply failed to deliver what the target audience wanted and expected to see. Software like Aperture (Apple's best software product ever, in my opinion), which could have served as a foundation for making Apple a truly dominant imaging/photography platform (including for consumers), was clumsily abandoned. Long-awaited products like the Apple TV were released with glaring issues, many of which ignored real-world concerns and some remain to this day. I of course could go on.
In addition, Apple's product lines are becoming increasingly confused and confusing -- one of the issues that nearly destroyed Apple just before Jobs returned. Who are the computers in Apple's laptop line targeted toward? It's increasingly unclear. What standards is Apple pushing? Lightning, one might have thought after the iPhone 7, only to see the laptops include a headphone jack and appear to advance other ports. Also, contrary to much popular mythology, Jobs was a proponent of industry-standard technology (particularly ports) where Apple's "invented here" offerings didn't offer a clear advantage. It was Jobs' Apple, after all, that championed USB as the replacement for several legacy ports when Apple released the original iMac. Cook could have done the same thing by using USB-C instead of Lightning on the iPhone, and this would have eased much of the headphone jack concern (because any new headphones people had to buy would at least rely on what Apple was advancing as an industry standard, available to all). Jobs also priced Apple's products fairly -- there were many instances in which the market was favorably impressed by a product priced lower than expected (the original iPad was the best example of this) and even where an "Apple Tax" existed, users understood why they were paying more. Not so now. And those things that Jobs didn't do well -- the cloud, for example -- have fared no better under Cook.
In my view, Cook probably was the right person for the job when he was hired. Apple needed someone who knew how to balance stability with structural change while positioning Apple for a sustained "big company" future. Cook did that. Now, what Apple needs is quite different: it needs a restoration of its "startup" ethos and it needs excitement. And it needs these things quickly: a widespread defection from a significant part of the "ecosystem" is now possible and would be extremely damaging. I believe Apple should celebrate what Cook has accomplished and respectfully show him the door.
