Rust doesn’t even have a real GUI library yet, let alone support from common game libraries. C++ supports a hundred times as many compilation targets and platforms.
If you are hardcoding anything platform related into your CML, you are doing that wrong. The default path must not be anything other than build and usage requirements. To support more usecases with CMake, you have to write less of it.
Games would probably be the biggest, most common scenario where C++ isn't going anywhere any time soon.
More generally, if you need to use existing C++ or even C libraries, Rust is at best an awkward fit unless someone else has already built and is actively maintaining the necessary bindings.
I would not want to be the one writing that CMakeLists.txt