If you are looking at something so far (40 years?), the smart device could be something you install right into your eyes and ears and you turn it on and off or switch modes through mental concentration. That is just the physical manifestation.
I cannot begin to imagine what the content side will be. Technology changes so fast and so drastically that it is difficult to think about it now.
I remember the days when the only mobile thing was the pager. First you get just a telephone number that you call back to. Then you get an alphanumeric display which allows you to send a very short text message and it was a big thing, couldn't imagine it. Then came the first Motorola cellular phone, the 大哥大 (the brick). SMS came in later, first read only then both ways and you thought you had the ultimate phone. The phones got smaller and smaller and technology is measured by the degree of miniaturization and silicon and multi-layer technology. Then someone brought in ergonomics. The phone must fit the hand but since with the acquired miniaturization technology, the phones looked quite empty inside, so new chips and features were added to fill up the space until the concept of a smartphone arose that required more than a one, two or three lines display.
Then began the debate on screen size with one school sticking with ergonomics and using software to deal with the size issue while the other school focused on hardware upgrading. The idea of tablet is born to allow more liberal surfing and AV capabilities. Then there is an attempt for a cult to have a compromised device that sits in between the phone and the tablet.
What I am certain is that technology that finally becomes the mainstream will not be something that is just one-up on the competition. It will be a revolutionary idea to capture the imagination. The cloud is one such idea and eventually everyone will adopted it. Apps was another such idea. Apples, Androids and Windows all have it in varying degree. People are always hungry for new uses.
Where will we get from here? Your guess is as good as mine.