From the article:

  With Visual Studio 2015 Preview you can target Android and edit-compile-debug regardless
  of your choice of programming models:

  JavaScript (or TypeScript) with Cordova, C++, or C# with Xamarin.

I wonder how much effort it would be to support C# as well, without Xamarin. Would they ever do that? Or would they prefer to not hurt Xamarin (or maybe Google/Apple) too much by offering their own C# crossplatform development?

I don't see what they would gain from a separate initiative when Xamarin has done all that work. They're "partnered", after all.

any famous apps done with cordova?

Wikipedia's app used to be. Not sure if that's the app they still run across all the platforms though. https://github.com/wikimedia/WikipediaMobile

Thanks! I must've missed it

Xamarin. It is a good solution in order to make a small applications (let's say a showcase/order app for a small restaurant) on iOS and Android at the same time.

For more ambitious apps, IMO the overhead will never be worth it.