I'm going to be a bit nit-picky, but C# isn't a vendor-specific language. There is also the mono implementation which is very very good, and doesn't lag too far

I'm going to be a bit nit-picky, but C# isn't a vendor-specific language. There is also the mono implementation which is very very good, and doesn't lag too far behind the .NET implementation. Xamarin is a decent example of writing C# and targeting multiple platforms. That said, there are better languages for cross-platform development, but that might not be a concern for a large number of companies and people. Locking yourself into a single language is the wrong thing to do regardless, and is what most of the answers in the link suggest.