To invoke the RIIR train, probably a lot of fertile ground in being the definitive Rust implementation of "solved" foundational libraries: zlib, libpng, libjpeg, etc. Something ubiquitously used which has very minimal/no churn. As Rust usage grows, dependency on the original C implementation will diminish.
It will diminish, but never go away until POSIX foundations or graphical programming standards get replaced (most of them defined via C ABIs).
When I was mostly doing C++ and C on day job, 20 years ago, they were the languages to go when doing any kind of GUI or distributed computing, nowadays they have been mostly replaced for those use cases.
Yet they are still there, as native libraries in some cases, or as the languages used to implement the compilers or language runtimes used by those alternatives, including Rust.
Even those change substantially over time, even if they're not directly rewritten, things get updated and relocated.
It's like how the streets in Rome have been the same for much longer than many of the buildings have been standing, even though the buildings are hundreds of years old.