Ok, so what is the negative? MS would be losing sales against the Steam store, ok it makes sense. However on the other hand they would be getting a bigger audience into their ecosystem, and that has a price specially when the opens the door to convert those customers into GP subscribers.
This is just a rumor and we lack of any detail it might include if it was real, e.g: Microsoft titles not available at the Steam store, or any other concession as Steam did back in the day with NVIDIA to offer Steam in GFN. Only a limited amount of titles are available on GFN while other publishers are against this, but in the end Steam supports GFN. Also, Steam would expand its customer base, those people not familiar with PCs, by leveraging the XBOX ecosystem.
For us consumers this would be a win-win situation, and that is what matters most to me as a gamer, there's no con on this if true.