Even though MS is a large money making behemoth, I don't necessarily think C# would be served any better by MS trying to support other platforms. A large part of .NET depends on a well supported class library. These class library can be very operating system sensitive. Backslashes vs forward slashes, directory separators, nonblocking IO, GUI stack etc. It is hard to do this well. Particularly on a platform that belongs to your competitors.

Microsoft has pushed Silverlight as a .NET vm that runs on Mac, with a smaller framework library. So far, this has met will little traction. I don't think it will encourage MS to push this further and wider.