They just spent $70 billion on Activision/Blizzard, sustainability doesn't exist and they don't care about it, and they don't need to. Microsoft's full weight is behind Xbox now, and they will keep pushing, and pushing, and pushing until overall market dominance.
This isn't about sustainability, it's about loss leading until the eventual overcoming. They could lose money forever and it wouldn't matter, they have a near infinite supply of it.
Their goal isn't to compete, their goal is to spend and grow to a point where there is no competition.