With the .NET platform now being available for cross platform development I can't see how there could be a decline in C#.
You'd have to be an idiot to use .NET for cross-platform development. In five or so years, Microsoft will discontinue cross-platform support, giving some BS excuse, like "no one was using it." What will the cross-platform developers do then, rewrite their code in a language that's really cross-platform? Doubt it.
As I've said here, before, this is just more Embrace, Extend, Extinguish.