The inevitability of some services to support textual commands is mainly given by the advent of wearables (displays disappearing, as in the movie Her). As you said, someone is already using Siri and on the Apple Watch that is indeed the best way to access some functionalities.
I don’t think all GUI apps should disappear, and I wouldn’t enforce apps to have a chat-like UI. But some should definitely enable some user journey through Siri. For this reason I think the solution is more like a SiriKit where apps can be used, even if partially, also through a Natural Language User Interface.