Anyone who has been around long enough (and not really that long all things considered) knows better than to ever trust Microsoft. Their whole model has been, and always will be: "Embrace, extend, extinguish".
So Microsoft is opening some things up? They've had no choice as they are being left behind more and more. They know the modern developer is not as interested in an expensive IDE and MSDN account when there are many alternatives. We are creating so much more software than before and the top down model Microsoft sold has become niche after many years of dominance.
Any open project Microsoft gets involved in and any open standard they get involved in should come with a warning label. Why? Because this is in their DNA.