In principle Rust could replace every line of C++ code in the world. The questions of how often it would be a good idea to do so, practical to do so, is harder to say. It is promising that this bluetooth stack only needed 4 lines of unsafe though!

Since the interop is zero overhead doing piecewise migrations is certainly possible, as has been going on with firefox, and curl and discussions of doing it in linux as well. You do complicate your build system and there is a non trivial amount of work to stitch the two languages together.