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They seem to be gradually bringing their gaming offering closer to Windows as well (game streaming, shared subscription, etc) and I'd guess at some tie in with MS365 accounts at some point (extra storage maybe).
I suspect they may be less worried about sharing with Sony than the impact of Google Stadium or whatever it's called taking off. If they can keep that relatively unpopular by offering a subscription service themselves then there's another benefit for MS.
Not sure why that matters apart from that it's great for the consumer.
Microsoft are the most cash rich company on the planet right now so...