I think one key point of the discussion here is that Rust provides a thin runtime which is close to the lowest-common-denominator OS API (in this case POSIX's byte-oriented IO), whereas C# provides a thick runtime which is close to programmer use cases (in this case structured IO). As much as I dislike this dichotomy, this is the textbook difference between a low-level programming language (annoying but predictable) and a high-level programming language (comfy but uncontrollable).
Now, of course, we could dream of an ideal world in which our programmer comfort would not be disturbed by crufty API design from the 70s. But if we cannot get that, the next best choice is to cater to both application devs and system devs using different tools. I would say that this is why having both Rust and C# is a good thing.