I worked on a project where we had Unity source access, indeed it is written in C/C++.
C++ is useful but having access to a scripting language would be nice. C# is a very nice language with very good features, javascript is also becoming the most popular programming language in the world (if it is not already) and I would seriously help bringing new talent onto a project.
Ideally there should be some mechanism to use whatever scripting language you want beit LUA, Python or whatever.
I disagree that C# is the “future” though - it’s pretty much tied to Microsoft and we’re seeing less MS popularity not more.