With C#, it's not _just_ the language. Typically, you'll pay for licensing costs of hosting the infrastructure required to run production grade C# applications. With Java, you can use open source infrastructure and servers.
That is only if you use C# with .Net framework over .Net core
.NET Core is still pretty immature though. The quantity of third party libraries that support it is tiny in comparison to what you get with Java.
But C# is still a better language that isn't as widely used (therefore it's underrated). And that's what we're talking about.
From what I can tell it is mature enough for stack overflow to use it
I was referring more to the overall ecosystem than the core platform, which is solid. Stack Overflow uses Dapper rather than Entity Framework, and that alone makes transitioning much easier. EF on .NET Core still just isn’t as good, and the third party library support isn’t as good either. For most people, I don’t really think moving to Core is quite worth it yet.
Stackoverflow runs on .NET Core? That's news to me.