Choosing Rust instead of C or C++ for new projects is a rather light decision. But introducing it into or outright replacing legacy codebase with it is a rather phenomenal undertaking. Fish shell was completely rewritten. Linux is introducing it in no trivial way. I wonder if the woes with C/C++ is that bad.
Librsvg did the rewrite incrementally, so you can choose to only use rust for new code in an existing codebase.
Other than having to know Rust, adding Rust to a C code base is not difficult. They play well together.
There is no need to rewrite old code but, once Rust is there, you are free to.
Linux is a bit of a special case as you cannot just blindly use the Rust standard library.
Having to have a Rust tool chain to build with may or may not be an issue.
For some use cases, like BSD or the Linux kernel, platform support is also a consideration.
