A) From my equally anecdotal experience, non-HTML GUIs are going the way of the dodo as a business investment. So, "primarily useful for developing GUI applications" citation needed.

B) Modern MS-RSL "reference source license" is now OSS compliant. The remaining difference between .NET Full Framework's Reference Source and what's happening with .NET Core is .NET Core has moved development to the open and Github as primary development platform. .NET Core is the future and .NET Full is "stable".

C) One example: http://referencesource.microsoft.com/#PresentationFramework/...

There's nothing stopping you from using the Reference Source today to build your own WPF fork (or, let's face it, you probably mean SWF). You can even start with Mono's existing mostly-there cross-platform SWF or Moonlight codebases and save a bit of time.

You know why you don't hear about lots of people doing that? I'm fairly certain it is because no one is building cross-platform GUIs in anything but HTML(5) these days.