Opening the source solves what problem, exactly? The only 2 arguments I've ever heard are: 1) More eyes finding/fixing bugs. 2) All software should be free and open source to anyone.

There are counter-arguments to the above, which I won't get into here. What I will say is, although Microsoft is a walled garden, it's a very large garden at that. If you want source code, go with a Linux variant. If cost really is a problem, there would hardly be any Windows Server users at all.