Interestingly, if you'd like to check out how some of these features were implemented, you totally can. Roslyn is open-source (http://roslyn.codeplex.com/).
That being said, I hope that all the open-sourcing is going to make it available where I really want to use it - Unity, for instance. (Disclaimer: I'm an open source dev @ Microsoft)
Nah, it won't have a monopoly. UE4 is a big deal for a lot of reasons, particularly its graphical quality. It's not the friendliest for indie devs at the moment, but it's been evolving and improving very rapidly.
And we'll see how long it takes Unity to actually support the current C#. My guess would be a couple years, since Unity 5 isn't even out yet, and there's usually 18 months between major releases.
That being said, I hope that all the open-sourcing is going to make it available where I really want to use it - Unity, for instance. (Disclaimer: I'm an open source dev @ Microsoft)