Ios development is a walled garden. C# is barely used except to target Windos. Go? Too early to say. If it did fade away, who would miss it, really? Its express design purpose was to be not powerful enough to make big mistakes in. Has it transcended that? If so, what is its purpose now?
Java has shown staying power, despite its shortcomings, although its connection to the Apache Foundation ("where projects go to die") offers just a ray of hope.
The Unity game engine use C# for scripting; I wouldn't call that "barely used".
Anything less than ten billion lines counts as "little used".
How big is Unity?
Also Xamarin which if I read this [2] correctly owns a third of the mobile app market. I don't know whether that's credible but apps are a huge market.
It is possibly that could get you the 10 bn. It is quite certain that there has been more than 10 bn lines written on.NET Framework, probably far more, as for 15+ years it has been the dominant language for corporate apps built on the Microsoft stack.
On top of that.NET Core is growing much faster over time,according to their blogs, and it's cross platform as you know.
[1] https://www.google.com/amp/s/techcrunch.com/2018/09/05/unity...
[2] https://www.datanyze.com/market-share/madp/xamarin-market-sh...
I think also it's inaccurate to call any of these languages proprietary. They are all open source and if you wish you can fork them and make your own. C# in particular is now run by the dotnet foundation which is nonprofit with elected board seats, putting it in a better place for governance than many community driven languages.