I'll concede the Xbox One wasn't Ballmer's fault, but he still gets a fair share of the blame for WP. It was (or should have been) Microsoft's most important new initiative for years, Ballmer could clearly see that it was failing, and yet he kept promoting Myerson and giving him more power. A better CEO would have intervened.
> "Windows Phone"
You can lay that at Terry Myerson's feet. The several breaks in backward compatibility was a big factor in ensuring WP had no apps.
> "Xbox One (which halved its marketshare vs the 360)"
That was crappy decision making by Don Mattrick. He also had a hand in killing Flight Simulator IIRC.