Rust is a fantastic language for game dev. It just requires you to think completely differently if you want to have a good time. You can totally write the traditional game loop or my preference ECS (bevy as one example). ECS still let's you do the crazy mutations without having to worry about ownership. Each system can focus on the components they care about and you can specify the order they run. If you want to be reckless and mutate global state, then you need to explicitly decide between things like RwLock<T> and RefCell<T> for example. I'm not saying everyone should drop whatever they prefer to instead use rust. It's ok to stick with what you know or enjoy. Good games are agnostic to the tools used to make them. I just don’t think Rust deserves to be dismissed the way your statement does.

Every function has side effects and variables will need to be modified in multiple places in the same frame

We try to avoid exactly that, because it is what caused us man-years of bug-hunting and bug-fixing over our past projects. Our end-goal (that is still very far away...) would be to have the state from the previous frame and the user inputs, do only pure computations based on this data, and write out a new state before rendering the current frame.

We do use C++ though (because Unreal, and console platforms), what makes this extra hard, because C++ is a language for writing bugs, not for writing software.