If you define success for Rust as "everything is written in Rust", then Rust will never be successful. The project also doesn't pursue success in those terms, so it is like complaining about how bad a salmon is at climbing trees.
That is however how the Rust Evangelism Strike Force does it all the time, hence these kind of remarks I tend to do.
C++ is good for some things regardless of its warts due to ecosystem, and Rust is better in some other ones, like being much safer by default.
Both will have to coexist in decades to come, but we have this culture that doesn't accept matches that end in a draw, it is all about being in the right tribe.