It could be a net gain for Microsoft if the Windows developer community begins to embrace github. As long as the most talented in the community don't leave the Windows platform, it could lead to a more vibrant and productive Windows developer community.
I've been using git & git-svn for all my c# projects and for committing to Lucene.Net. The msys version of git has come a long ways. I also prefer using github when possible. Some of us do branch out and embrace outside Microsoft technologies.
Its possible to sway the windows developer community to branch out and hopefully more people in the community will take initiative to sway others to do so.