Note that of course Jai's tooling is also written in C++
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.
I mean, he's _allowed_. The Compiler Police aren't going to roll up to his house and take away his Jai compiler if there isn't a quorum of HN users blessing his efforts. But people can point out they don't feel the juice is worth the squeeze. Also, Blow is certainly an advocate for his position, which means this kind of public debate is germane to the question of if _other_ people should adopt Jai.
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.