Much of what can usefully be written in C++ cannot be expressed at all in other languages, Rust included. Thus, for serious users, switching would mean a big step down.

Most of the powerful, unique features are there to help encapsulate semantics in libraries, enabling libraries to adapt automatically to circumstances of use without loss of performance. As a result, libraries in C++ can be more powerful and useful. The more a library is used, the more resources are available to optimize and test it, so libraries can become extremely robust.