I think it depends on how you define mobile. You're right when you say that a tablet or smartphone screen isn't suited for word processing, programming, ... but when you connect that device to a 24" monitor, a keyboard and a mouse you got yourself a full blown computer. Just unplug it and it's mobile again. I believe that is the future: everybody carrying his 'personal computer' in his pocket to be used everywhere in any form factor.
Sounds awesome, no?
So instead of mobile taking over the desktop, I would say it will be more like a merge of both.
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