Why? Out of curiosity.

The title seems to have changed from something like "now it is rewritten from Python to C#". I now suppose all the downvotes of my comment happened after the retitling.

Why is it a "sad day" exactly?

It's like investing energy to switch from "My app runs on Kubernetes" to "My app runs on Windows XP" and being proud about it for unexplainable reasons.

It's actually not like that at all since C# is a lively, rapidly evolving platform and also probably friendlier to a large codebase than Python (and almost certainly more performant).

And how good is it at avoiding a large code base?

I don't think good C# code is actually much more verbose than Python unless you're stuck on a very old version.

It is compatible with F# which it pretty expressive and have a nice and clean syntax.