Maybe - but in this case, Ruby is written in C, it uses C extensions when performance matters, but tooling for the Ruby language itself is all in Ruby. Rust isn't replacing the use of C in the core of Ruby (yet) - it's stepping in to the area where Ruby would have been traditionally used.
Similar thing is in motion with the JS toolchains. Rewriting in Rust is easier than rewriting in C, but why didn't they previously rewrite in something like C++ or Go? I'm guessing because people were simply not interested.
