With all due respect, you suffer from failure of imagination. As technology improves, I can just as easily envision a completely digital future.
"What dream of the mind's eye will remake the world?"
-Carl Sagan
Quite honestly, you are the one failing to imagine here.
In a digital future, there still needs to be a mechanism to convert your thoughts to a digital medium. Maybe one day computers will be able to read your thoughts, eliminating the need for another interface. But that seems somewhat unlikely to happen before today's schoolkids become adults. So there is still a need for some sort of interface.
Right now, the keyboard and mouse are the most common interfaces, with the mouse being replaced by touch screens. And sometimes the physical keyboard is replaced by the digital keyboard on a touch screen.
The other common interface is voice recognition. But there are lots of instances where voice recognition is not appropriate, especially in areas where you don't want to make noise, you want to keep information confidential, or too many people using voice interfaces would be disruptive (like a classroom setting).
But right now, pretty much the only quiet interface is some sort of keyboard. And as good as keyboard are, they are somewhat cumbersome, space consuming, and require two hands to operate efficiently. A keyboard may be fine if you are typing a long paper or you're sitting down at a desk to do work. But for when you are on the go, or just need to make a quick note, it is far from ideal.
But imagine. What if there were some sort of interface that took up less room, that only required one hand, that you could use anywhere? That interface exists in handwriting. With improved handwriting recognition, you wouldn't need a keyboard. You could use the whole screen on your tablet or phone .... or better yet, maybe you could use your finger to write in the air.
If you want bigger fonts, just write bigger letters, If you want to draw a line or circle, just draw it. Handwriting gives you all sorts of flexibility that is not so easy with a keyboard.
Just imagine.