Well, I don't know about the cat command (though that _is_ appropriate for something like the Redox OS, which aspires to be a pure-Rust desktop operating system

Well, I don't know about the cat command (though that _is_ appropriate for something like the Redox OS, which aspires to be a pure-Rust desktop operating system). But there is a broad-based desire to rewrite libraries in Rust for a couple (IMHO valid) reasons. One is the API. While it is possible to have a nice and idiomatic Rust API to a C library, it will often be nicer to use when it is designed from the ground-up in Rust. Traits, interfaces, etc. It is also often desirable to avoid C when cross-compiling (such as for embedded systems), as that is generally a hassle to set up and get working seamlessly.