Asking in 2020 what will replace the smartphone feels like asking in 1950 what will replace the motor car. Augmented Reality contact lenses are the modern equivalent of 1950s predictions of nuclear-powered flying cars. I don't think smartphones will really be replaced for forty years or more. It sounds like a long time but it really isn't. We've not come that far since the original iPhone. And that itself wasn't that…
I don't think we really do. Constantly pulling your phone out of a pocket is unnecessary friction, and just as pocket watches gave way to wrist watches, I think phones will become something you wear, not carry.