Having worked on .NET frameworks up to 4.5, and then vaguely following the evolution of C# since then, it certainly seems like a "new language" in many places.

The trouble is that you can still write code in the original language, e.g. don't use the elvis operator or Linq or '?.', which is what is likely to happen if you write a lot of code in other languages too - C++, Java, Kotlin, Python, Javascript. There's a common 'subset' language in all of those that's easy to carry mentally from one to another.

One thing that Intellij/Kotlin has is that if you write "old fashioned Kotlin", there are a lot of IDE inspections that can convert that "old style" code to sort-of idiomatic Kotlin.