Might be grass is greener mentality, but I think I prefer this to whatever C# is doing.

The sealed keyword makes sense, isn't unique to this feature and you can pattern match exhaustively. I feel like I could explain this to a beginner without overloading them, it's still 'just Java' while giving you a way to occasionally model a one-of relations with out the awkward default handling while sticking to the predominately OOP driven nature of the language.

The stuff you can do with a functional language is amazing but I don't think bolting on new paradigm's until you have a diabetic Frankenstein of a language is the right answer, Scalar is right over there if that's your preferred style.