While I agree that Rust has advantages for performance-critical work, I don't think "systems" implies performance criticality. In the Unix tradition, "systems programming" seems to be a slightly blurry term for writing the basic tools that come with the system, not necessarily just the kernel and such. I think the term is supposed to be in contrast to "scripting", but I'm not sure the duality is that strong. For example, USENIX '91 had a paper "Awk As A Major Systems Programming Language", which implies that implementing tools such as 'nroff' (one example used in the paper) counts as "systems programming".