> He did contribute to MS before he became CEO.
I thought we were assessing him as a CEO, not an employee. Sure, he contributed before, and during, being a CEO. And obviously everything is speculation since we can't A/B test hypothetical timelines. But the numerous anecdotes of his "unique" management (if some of the stories are true, which I cannot verify, the guy should get sued for harassment on multiple counts), the fact that he missed obvious opportunities for MS (sure it's easy in hindsight, but even then I remember thinking "what is he doing?"), and the fact that Microsoft's reputation plummeted during his reign (again, subjective, but none of my friends wanted to work there and openly mocked it as an employer and that has completely changed under Nadella) are all strong signs that he wasn't an excellent leader.
But ultimately this is just my opinion, and he's a brilliant (I think he's a horrible CEO, but a very talented mathematician) billionaire. So what do I know?