Yeah, you can totally use both gdscript / C#, as well as C/C++/Rust for targeting performance sensitive objects, like doing heavy simulations, since gdscript / C# will still be often simpler for glue code and light-weight signal handling. I think people misunderstand that this is less Rust bindings for the engine and more so like producing plugins/mods for the engine to import.

For why Rust vs. C++, Rust is often able to handle embedded invariants through the type system, while in C++ you'd need to juggle them in your head, which scales a bit poorly through time or number of people working on the project. That said one thing that doesn't match well is the Godot is very inheritance based, which is a bit of an impedance mismatch for using Rust, but I've found gdext to translate these things into composition well enough.