One school of thought is that it’s defective hardware design if well-behaved software is all that’s keeping hardware from catastrophically failing.

Often the line between hardware and software is blurred (is FPGA logic or processor microcode hardware?). Avoiding catastrophic failure may be a non-trivial problem. In the example case of a CNC machine, you must avoid putting too much strain on a tool, or suddenly colliding with the workpiece, thus requiring knowledge of the workpiece geometry. Software can also be easily updated to mitigate hardware faults or design problems (e.g. processor bugs).