Rust is a nice language for greenfield system software project. But everyone in the software industry knows that you can't just put a project on hold and rewrite completely in new language. I can't see anything good coming from this initiative.
Rust is designed to integrate into legacy codebases so security critical code can be rewritten in small steps. See the URL parser I Firefox. Moreover, the initiative this post is talking about uses rust to rewrite small parts of a C codebase. Honestly, I'm getting more and more disappointed by the quality of discussion here on HN.
There is no such thing as security critical code. If it is out there - it can be expoloited. There are a lot of stories with minor exploit chaining leading to huge compromise.