Absolutely true that the JVM gets more use. But it's also true that the present situation is never set in stone. Remember how popular it was to malign Java back in the late 90s?

And it's also true that the CLR really is technically superior to the JVM on a whole slew of points. It has runtime-level support for generics (meaning C# lets you do all sorts of nice things with them that Java doesn't support). It has user-defined value types. It supports coroutines (i.e., 'yield'). It supports anonymous functions and closures. For those who just can't leave pointers alone, it even has unsafe blocks. And so on.

So while it's definitely not "proven" in a sense that will impress your average PHB, in terms of technical merit I don't think there's really any comparison. Mono wins hands-down.

If you want "proven" you should probably be using NodeJS then... not for GUI apps, but for headless apps and services... since it's core engine is V8, which runs on nearly a third of the desktops out there with far more regular use than Java. And the language JS, with more developers using it than just about every other language combined.

This is kind of tongue in cheek, but seriously... NodeJS is relatively new (since 2009) and it's gained a LOT of traction... it isn't the perfect solution, but then again nothing is. I like Mono and have used it, they've worked through a lot of the earlier issues, and it's pretty decent at this point... IIRC it's the core of the Unity tools, as well as having Xamarin tools available to support the majority of mobile platforms.