I actually think they will need to eventually have just one functional language (doesn't mean, and I expect them to, that MSR won't have post F# languages). Something that most likely replaces both C# and F#. I don't forsee wide adoption of "two functional languages" based solutions. Folks generally want a one stop shop. It's possible to argue that C# + XAML is a two language solution, but in my experience bosses don't see it that way. Whereas if I where to propose a solution based on C# with a "dash of F#" I would have a hard job justifying the additional complexity cost.

No, ultimately I think MS need to cap off C# before it ends up in the mess of language corner cases that C++ has decended into. I'd much rather look back on C# fondly than have too many disperate paradigms bolted onto it until it starts looking like a Vogon Construction Ship.