Sometimes, but when it's a matter of life and death, when it's a matter of whether you will fire your missile or not and whether you'll hit your target, when you want to keep traffic flowing...wouldn't you choose something that increases the chance of having the right answer rather than a small performance increase?!

This seems like very much the wrong concern. If you're writing a PC or console game, sure! If you muck up, nobody dies. If you are doing anything really important, like you're describing above, you should pull out all the stops on safety, whether it's in Rust, or adopting the very best practices possible in C++20.