Sun is a beautiful case study in what happens when you're a little too aggressive about the "openness" mantra. They bet the farm on SPARC and lost. If they had kept pieces of Solaris closer to chest, who knows what would've happened, but they would've at least had one more potentially-viable commercial offering.

I assume that Microsoft is not going to be naive enough to do the same.

IMO Microsoft's interest in openness revolves around remaining a viable option in the cloud era. They need to protect their investment in the .NET ecosystem and MS-centric development workflow, which means making it easy for people to run .NET programs on *nix-based VPSes and containers.