They could also use C# instead of UnrealScript on top of C++. This way you would be able to leverage the Visual Studio while getting one of the best languages. It would make Unity3d guys welcome once they need something better as well.
To second what scott_s said, you'd have a huge interop layer. The fact that C# has a closer match to C++ types and primitives doesn't mean you have no interop. Unity has a huge surface area between the C++ parts of the engine and the C# parts of the engine; a ton of work goes into maintaining that, and it complicates porting to new target platforms like the web (can't just feed everything through emscripten).
Yes, C# and other .NET languages are nice, but the open source implementation (Mono) ends up being difficult to port to novel platforms due to a combination of technical and legal issues.
PS: I like the BluePrint though.