I am no expert, but I am under the impression for HFT that high performance is the absolute goal and slight percentages in performance can affect profits.
HFT entails only a small part of the usage of C++.
It's still being used in many embedded systems (including automotive, aviation and building automation), where Rust is also available. Interop is always possible with C/C++ so there's little excuse not to use Rust there.
