With Microsoft open sourcing its ASP.NET framework, and the fact that a replacement for the Internet Explorer browser called Spartan in the works to be shipped with Windows 10, radical change might be coming faster and deeper than anyone can imagine.
I believe that Microsoft might continue to keep making more of its software open source, yet I don't think that it would open source all of it. Ideally, Microsoft should make the move completely to open source. The move would be greatly beneficial for Microsoft itself, the users of its products, and the open source community.
The idea of having Windows becoming open source, is one that can really makes me happy and gives me some hope that it would make easier for open source software to reach more people. Having Windows as an example of open source, would make a lot to help people to trust open source software in a way they don't do at the moment.
What's more, turning software from proprietary to open source seems to be a path that many companies are taking because the benefits that comes with taking such a step. If the trend continues, it isn't that far fetched that Microsoft would take such a step. Profits aren't something that go against open source, if the software abide by the the free software definition.
Time will say how far, and deep, will Microsoft take those changes.