Given that Windows is a considered a "service" it makes sense unless there is something to hide (some odious telemetrics, perhaps). No one is going to replace Microsoft in selling Support and Updates, unless it isn't some niche that it doesn't care about anyway, just like Oracle didn't destroy the RHEL market with their own Linux.

I'm thinking it can help them win some government contracts outside the USA (by saying "you, and anyone else can see that there aren't any backdoors"). The potential open-source community can expand Windows markets the same way it did for Linux, making sure Microsoft remains relevant.