You don't know that the "new version failed." It looks more like incompatibility at the system interface level, remediable within hours (as happened). Flipping back to the Last Known Good may not be an answer in the larger scheme of things. Boeing had to fix the problem with the 737 MAX. It couldn't just throw up their hands and fall back on the production of 737 NG models; the NG couldn't address the market that the MAX was designed to compete in.