Linux

but I do think a sizeable portion of existing C++ devs who don’t want to use rust exist

That may be true. But out of that pool of people it seemed that very very few wanted to work on the fish project. So it was not helping them much at all. The is a vastly larger pool of people that don't want to learn C++ and some of those may be willing to pick up rust. It would not take much for that to out number the number of C++ devs that want to work on fish that also don't want to learn rust. Given there are not a huge amount of contributors that regularly contribute to it according to their announcement blog post.