For me,

1) Linux and jetbrains rider

2) 30k sloc heavily v&v'd code? Why? The cost and effort would be insane, and, to MS' credit, the current .NET ecosystem is great.

3) First class citizen. Rider (and the other jetbrains stuff) is as good as any other I've used

In my eyes, as someone who has been exclusively a Linux user since 1998, I now include C# in serious consideration for greenfield projects as well as legacy.