There are mainstream efforts to do the legwork to allow Linux kernel modules to be written in Rust and a fair few core system libraries (the most obvious example being librsvg) and applications have undergone Rust rewrites. And of course there's Redox OS which a full OS entirely written in Rust, and it only contains a few hundred lines of unsafe code (which, given how Rust works is the only really key code that needs to be audited for memory safety).

The reason that more things aren't yet written in Rust is because these things take time, and there is lots of inertia to switching languages for established projects (one big roadblock to such rewrites is that maintainers need to be familiar enough with Rust).