Don Mattrick has a lot to answer for. Microsoft was killing it during the seventh generation and he was able to burn everything down over a period of two days. Xbox never recovered after that.
Don Mattrick's mistakes were near-fatal, but Phil Spencer's done more than his equal share of torpedoing the Xbox division. The blame at this point can rest squarely on his shoulders.
Well, him, and the person who refuses to replace him.
Mattrick's missteps were not just about two days of revealing the XBox One, but really the entire second half of the 360's lifespan. The focus on Kinect to chase the Wii audience and the shuttering of many of their development studios left them thin on the exclusive front, which became a huge liability as they transitioned to the next generation.
But Spencer has a lot to answer for. He has been acquiring studios for 7 years now with little to show for it besides Obsidian having prolific output. Game Pass is flat, Xbox Cloud is flat, and their hardware is being outsold by Sony month over month, year over year. They seem to be pivoting to "Play Anywhere" which feels like the first phase of a three phase plan to retreat to the moat of Windows, where they can use Steam's momentum to ensure that games will at least appear on the platform. He inherited a mess but has made plenty of his own mistakes as well.