Who says that C++ didn't break through that barrier, though? Sure, the big popular OS projects like (I'm assuming) Windows and Linux mostly stick to C in their kernels, but Haiku (for example) seems to be doing quite alright being written predominantly in C++, including in the kernel: https://git.haiku-os.org/haiku/tree/src/system/kernel
Outside the kernel, C++ seems to have more significantly broken through that barrier.
Outside the kernel, C++ seems to have more significantly broken through that barrier.