As to why Rust, there's actual historical information about why Graydon wanted to make it and why Mozilla decided to fund that work after it was started.
Jon has said the intent was that Jai would allow him to make more games, because C++ held him back so much. So, yeah, it would need to be superior to C++ in at least this way, and by at least enough to justify the effort expended, writing C++, to develop Jai.