At the moment I just write bits in C whenever I need it but when it scales in a decent size I would just write the whole thing in C++/Rust

C++'s problem is that most people never learn it properly and think "C/C++" is a thing because most courses/tutorials are stuck in the 90s and effectively teach shitty C with classes and iostream. Rust is great and has really nice features which I'd like to have in C++ too (like pattern matching) but memory safety is really not an issue in proper modern C++. I'm aware that this is a bit of a "you're just holding it wrong" but pretty much all languages have things you shouldn't do anymore once they evolve, it's just more obvious in the one with strong backwards compatibility requirements to the ancient language created before all the modern research into language design.

    std::vector<int> v {1, 2, 3};
    int& x = something(v);
    something_else(std::move(v));
    x = 42;

Honest question as I'm not a C++ developer: Even with all the new C++17/20 stuff?

All the things Rust advocates insist cannot be done safely in C++ are now trivial. Use-after-free, leaks, buffer overruns, what-have-you, no longer need any attention to avoid if you stick to modern C++. Some people insist on coding as if it were C, and bring along all of C's failings. Leave them behind.

Those people that insist on coding as if it were C are still a very big group across the industry.

I mean, maybe. Or maybe it gets replaced by something else.