That example is really difficult, because between the lines it sounds like it has been working sufficiently well despite the cowboy style, substituting methodical error-avoidance with that special kind of brilliance of super deep code knowledge and remembering all the pitfalls. Very difficult situation.
Regarding "rewrite in Rust", I believe that it's a smaller part of the appeal of Rust than it appears based on the amount of code actually written: the main excitement is from people who never really ventured from heap+gc languages into manual memory management but who love the idea of being able to write gc-less native code. And many of them would rather write it slowly, one borrow-check at a time, than with all the memory bugs they created dipping their feet in naive C/++. Those people rarely write much Rust, but their excitement infects some in malloc/free land and for them a rewrite is super attractive because it skips the entire explorative part of software development that is really not the strong point of Rust.
(yes, this is pure projection, not only was I describing what draws me to Rust, I also failed to pick up on the codebase of my previous boss, too much "how can we make this entire thing less slow and error prone" and too little "cram in requirement X, even if it might turn out to become requirement Z because it pushed the codebase over the edge to terminal unmaintainability")