The rationale for using Rust over C that I see published the most is "memory safety". But I can write small C programs for text-processing using flex that do not manually manipulate memory. What benefit would there be to write them in Rust.
I've written more in C than in any other language (and I've written in dozens of languages professionally over the past 25 years). But the one thing that gets hammered home with every line of C I write is this: You're playing with a loaded gun.
The more code there is, the more likely it is that you have subtle heisenbugs that break in mysterious ways. My current job requires C (for a number of good reasons), and I really notice the jump in crazy, hard-to-track bugs despite my DECADES of experience as a careful, expert C programmer.
C is "simple" because all of the complexity gets pushed to the compiler behaviour and runtime environment. You don't even discover how many assumptions you've been making until you have years of experience under your belt.
Previously they would write in something like C/C++ or use the JVM but try to avoid all garbage collection events though very carefully avoiding allocations, etc.
For them, Rust is a gift!
Also: memory fragmentation is only an issue if you don't have a proper memory management strategy (which you absolute need in any non-trivial code base, even in GC'ed languages).
...which unfounded assertion would that be? As far as I'm aware there are no contenders which would even attempt to throw tools like Maya, 3DSMax, Blender, ... from their throne. And all those tools hail from the 90's.
The only exception in recent time which gained some traction might be Figma for 2D design, but even though it runs in browsers the important parts are also written in C++.
Rust’s compiler is slightly slower than many other languages (as it simply does more), but it’s not a significant degree where you would need some monster machine..
This is easily solved by writing a large Pong application in Rust.