The new language features you mention are found in hot, new languages like Kotlin, TypeScript, and Swift. Some are being added to Java as well.

This implies that C# is keeping up with the needs of modern developers. In fact, C# was an early adopter of some of these features.

The real question is why someone would choose C# over any other language. You don’t hear enthusiasm for it outside of .NET and Windows, nor is that ecosystem expanding into new markets. Rather the growth is in mobile apps, web apps, and cloud-based virtualized services, where .NET is an option, but has little to recommend it to non-Windows developers.

This puts it in the category of a legacy system: not growing, but still with a substantial user base and set of applications that aren’t going away soon. Unlike C++, C# is keeping up well enough that nobody is proposing alternative languages. Indeed, Kotlin is not being targeted at .NET precisely because it is “much more modern, full-featured, and nicely designed” than Java. ( https://discuss.kotlinlang.org/t/kotlin-on-net-platform/2142/15 )

So C# is not going anywhere soon.