It was a sad moment when this great project stopped by MS.
So, it would be great for developers and MS if they open source it, and leave it for the community to continue its development.
This will take off the cost from MS, and leave a great product, that Windows/Xbox can benefit from, and might get re-born to another life that benefit MS.
also it would benefit to be a great big source code for game makers for the MS platforms.
So, i heavily advice MS to open source it.