Java is not regarded as convenient for web development. Fast when built, yes, but not convenient. And C++ is borderline unsuitable. (A well-chosen and carefully used subset, sure, but the exact things that Rust is fixing about C++ is also why you should think about a dozen times before using it to build web apps.)

And neither are as safe as Rust is/will be with multithreading. Doing really pervasive concurrency is possible, but not particularly easy with those languages.