I see what you're saying but I don't know if I agree on this point. Games are buggy messes before undergoing a long and arduous quality assurance process. While Rust's iteration speed is maybe not great, I don't think it's really significantly worse than C++ iteration time, especially not with efforts like subsecond, that bevy uses for instance. And Rust is much more ergonomic and convenient to write than C++. It is in some ways a higher level language than C++ while managing to still allow for very fine optimisations, which are sometimes required in games. I think this would translate to productivity improvements too, although I don't really have any data to back that up obviously.

I personally think the momentum is mostly what C++ has that Rust is lacking - i.e. a large body of game developers who are already well-versed in C++ and a large and established body of game engines and frameworks for every kind of game. Games are often pressed for financing, so venturing into the territory of new engines and new languages is not something most game devs can afford or want to spend their money on.