Something they should be able to do now, but do not seem to, is to allow you to train Siri to recognize exactly your voice and accent. Which is to say, to take the speech-to-text model that is listening and putting it into the Siri integration API, to both be 99.99% accurate for your speech and to recognize you and only you when it comes to invoking voice commands.
It could, if it chose to, continue to recognize all voices but at the same time limit the things the non-owner could ask for based on owner preferences.
It could, if it chose to, continue to recognize all voices but at the same time limit the things the non-owner could ask for based on owner preferences.