There is a reason GCC, LLVM, CUDA, Metal, HPC,.. rely on C++ and will never rewrite to something else, including Zig.
Looking at LLVM build times I seriously believe that C would have been the better choice :/ (it wouldn't be a problem if LLVM wouldn't be the base for so many other projects)
Same for the Metal shading language. C++ adds exactly nothing useful to a shading language over a C dialect that's extended with vector and matrix math types (at least they didn't pick ObjC or Swift though).
Yes, inertia. If those projects started today, they would likely choose rust.