Don Mattrick isn't the reason that so many Microsoft studios have repeatedly shipped poor products or failed to ship at all over the past decade. Don Mattrick isn't the reason reason that Microsoft still seems to have the worst taste-testers in the industry. Don Mattrick isn't the reason that internal studios like 343 have messed up 18/6 requirements. Don Mattrick isn't the reason that Microsoft kept acquiring, messing up and then firing studios, ending with a massive acquisition that can't even pay itself off for over a decade. And above all, Don Mattrick isn't responsible for the relentless drive for a weird fusion of unprofitable subscription service and cloud streaming offerings at the cost of overall game sale profits.
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.