I think it comes down to the fact that for all the « conversational » publicity stunt, the current implementations insist on the voice assistants to be non-moda

I think it comes down to the fact that for all the « conversational » publicity stunt, the current implementations insist on the voice assistants to be non-modal. When you think about it, every vocal interaction you have with people is explicitly (through voice) or implicitly (through body language) modal. How hard would it be for the mode to have to end with « thank you Siri » or a sensible time-out, or now with the iPhone X, loss of directed attention (and even activated again with return of directed attention within a reasonable timeframe)