My take is that ES is good for exploration and faster development but should switch to SQL as soon the product is successful if you're using it as the main db.

For the speakers like Alexa and Google Home, voice being the only input allows user to say whatever they want hence making the task space infinite. But the voice recognition and NLP is not in a place where it can recognize everything the user has said. This creates a less than stellar experience with the user having to repeat, rephrase or even worse abandon the task. I think this platform will blow up when NLP/AI is able to detect user intent with near perfect accuracy and is able to make the interaction with the user as fluid as with a well designed app. It doesn't hurt for Amazon to have a large installed base ready to use the platform if/when intent recognition becomes par.

Of course it will never replace phone/desktop as there will be things which we cannot say over voice (secrets) and where it is not possible (loud places) or just not courteous behavior.