While true, this blanket statement does a disservice to the actual situation. The parts of the language/CIL stack that make .NET important -- apps on Windows -- remains closed. It is only the ASP web stack that has been open-sourced, and can now be run on non-Windows platforms. When the WinForm libraries are open-sourced (and, perhaps, Visual Studio), then there can be an honest evaluation about ".NET" being open sourced. That's my opinion, anyway, as a 20-year Linux fan and current .NET Windows developer. I don't have enough rep to downvote anyone.
As a 12 year .NET developer and linux user I can't express how much I disagree with you. First because saying that nowadays windows apps is more important than asp.net is crazy talk. Most development is web nowadays and will just increase.
Second because what is being open sourced is not "the rest of the framework", but what is needed to build a new framework today, without the dependencies and problems from yesterday. The team announced that in time, even wpf and other windows stuff would be built on top of the new corefx as additional libraries.
The new corefx is not an asp.net only thing. It's just that asp.net is the most demanded and also the least dependent part of the framework to start this change.
There is still a lot of things to be done in order to achieve that, but I'm pretty sure it will happen because once things like collections and linq start evolving on corefx, they won't want to maintain two distinct frameworks.
Take a look at the most discussed topics on github, there is a lot of stuff being discussed there by the core team.
