At times, I regret DisplayPort not having something comparable (it has data channels, but nothing "as well specified" as CEC for controlling other devices). I think you explained part of the reason. If it were specified, it would require careful consideration, and probably a conformance test suite. I still fully expect manufacturers to try to re-brand it, extend it themselves, and generally botch their software like they end up doing most of the time.
At times, I regret DisplayPort not having something comparable (it has data channels, but nothing "as well specified" as CEC for controlling other devices). I think you explained part of the reason. If it were specified, it would require careful consideration, and probably a conformance test suite. I still fully expect manufacturers to try to re-brand it, extend it themselves, and generally botch their software like they end up doing most of the time.