The problem here is that that 5 years of C# is quite possibly spent working on the same codebase...that person with 3 languages either solved significantly different problems, or worked on different codebases.
While yes, the 5 year C# coder is probably a veteran of the language, they probably don't have the breadth of experience of styles you get.
I'll take a bloke with a year of C, a year of Javascript, and a year of Ruby/Lisp/Smalltalk over a veteran of a single language for three years. You learn to think differently about problems.
The thing is the guy that is new to the language is not going to be writing idiomatic code. In C# this means you are going to see unnecessary ISerializable implementations, problems with Data Binding/MVVM, fighting with the GC, and not knowing what you get out of the BCL. Depending on their personality and who else is on the project this may not be a problem, but without some mentoring you can end up with an unmaintainable mess very quickly.
I don't think you are giving the language enough credit, especially one as well designed as C#. I'm not a Windows programmer so I hardly get to use it, but I once tried writing a Windows Phone Application and I was surprised at fun it was to use C#. The plethora of resources available also made it simple to write (what I thought) was idiomatic code - not much unlike Go.
Now, this experience has left a wonderful taste in my mouth with regards to C#, and I wish I had more opportunities to use it. And honestly, how long do you have to be immersed in a codeabse to being writing idiomatic code. A month of hands-on C# is enough for someone to write idiomatic code, and I wouldn't expect someone to be fully integrated into a team in under a month, so it kind of evens out.
But there are always those who don't bother learning the language. Like one of my predecessors who more or less wrote C# like C++ (pre-C++98). My apprentice then tried learning the language from existing code and in turn took a bit of mentoring to write idiomatic code. I do agree that the language is nice and well-designed, but people can write crap in any language and googling around doesn't necessarily lead you to the authoritative or most current documentation but oftentimes to crappy tutorials, beginner forum posts and the like. It's not that hard to pick up bad patterns, sadly.
I'm not so sure about this. I think you'd be surprised by how rapidly some people can grok things, including things you might believe are only visible through extensive experience. What it takes is the ability to follow structural ramifications to depth, therefore being able to anticipate aspects of the system that others usually learn through experience.
While yes, the 5 year C# coder is probably a veteran of the language, they probably don't have the breadth of experience of styles you get.
I'll take a bloke with a year of C, a year of Javascript, and a year of Ruby/Lisp/Smalltalk over a veteran of a single language for three years. You learn to think differently about problems.