It makes more sense for new software to be written in Rust, rather than a full rewrite of existing C/C++ software to Rust in the same codebase.

Fil-C just does the job with existing software in C or C++ without an expensive and bug riddled re-write and serves as a quick protection layer against the common memory corruption bugs found in those languages.

Even without Fil-C I do not think it is even clear that new software should be written in Rust. It seems to have a lot of fans, but IMHO it is overrated. If you need perfect memory safety Rust has an advantage at the moment, but if you are not careful you trade this for much higher supply chain risks. And I believe the advantage in memory safety will disappear in the next years due to improved tooling in C, simply by adding the formal verification that proves the safety which will work automatically.