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It's just that the smartphone of tomorrow will not have a small screen. It will not only have a big screen, it will have a keyboard, a scanner, a printer,... atached to it.
When you get home or to your office you will place your smartphone in front of it and it will attach to all your pheripals just like your laptop, does today.
The smartphone will carry your identity and your local data and be your portal to the cloud. Why should you have a laptop a phone, a tablet all with a mobile internet plan of its own, when you could keep that processing and commuication power in a relatively small portable device.
Well, as a matter of fact, I actually hope that the smartphone of tomorrow will have a small screen, the 4 inch plus screens of today are actually a bit too big for carrying around comfortably, and if it was easy to connect to a 12 inch touch screen you could carry, and your large 27 inch screens on your desktop at home it wouldn't be a problem if the built in screen was just 3.7 inch or something like that.