As you know, Windows 10 is a "service", meaning that it is a version that will evolve overtime non-stop. It will receive feature updates semi-annually. Mainstream Support ends on 2020, while Extended Support ends on 2025.
So we need a new version of Windows that will remain supported forever. And the first one to be "open-source".
Recently, Microsoft has begun developing a next-generation Windows Platform, namely "Windows Core OS". This platform is like the Android Open-Source Project, which is the "bare-bones" for a proprietary Android operating system. Like the AOSP, Windows Core OS will not include any proprietary binary-blobs or software. Without those, the platform can be open-source.
Just to let you know, Windows Core OS is not for everyday users. It is for device manufacturers and developers to build custom Windows operating systems for their devices. This platform is "modular", meaning that a version of Windows can be tailored to a device's needs. To make a proprietary Windows operating system, you would have to inject proprietary binary-blobs and software into the source code.
Microsoft can still make proprietary editions of Windows, based on Windows Core OS. The "Home" edition could be proprietary freeware, while the "Pro" edition will be commercial software. The "Enterprise" edition will be subscription-based, and will be sold at volume licensing as usual.
With Microsoft's first open-source Windows platform, it could certainly beat AOSP, and could see device manufacturers give up Android on their devices.