Linux on ARM is a very mainstream platform from the perspective I’m looking at this from, I’m afraid. I’m talking about strange Unices running on architectures that GCC may or may not maintain a backend for, or maybe a BusyBox available on some debug interface, maybe a school project to build a simple POSIX-compatible OS or a bootstrap for a platform that had been recently jailbroken. In these cases C is almost always the go-to language and I suspect it will remain so for the foreseeable future as this is precisely the long tail of distributions that standards were intended to provide a base set of tooling for.
This is clearly not one of the examples saagarjha was thinking of, but an actual example where Rust is not an option is the x32 ABI for x86_64 platforms. This ABI works well with GCC, but LLVM suffers from multiple code generation bugs. As Rust is based on LLVM, Rust for the x32 ABI does not work well either. While it's possible to use a mixture of ABIs, so that you have a mostly-x32 system but with some x64 binaries, this requires a multilib setup with multiple copies of system libraries. A pure x32 system cannot currently have Rust utilities.
(I have some patches to improve things but I have not been able to submit them to LLVM yet, and with those patches I did manage to get a working x32 build of rg on my system. I hope to be able to do so in the future.)
https://old.reddit.com/r/rust/comments/cdcads/