Phones are humanity's magic crystals which enables everyone to send instant communication channel to anyone else and tap in the greatest common knowledge base ever. I don't think there is a reasonable future without everyone having his own personal device for this function. Maybe it won't called a "smartphone" but it will provide these functions.
It won't be going away for a long time though. It's here to stay a while.
The current form factor is a best-we-have compromise between ease of use, and utility. I think the next big thing will be the decoupling of utility and ease of use. AR/3D gesture tech in a discrete google glass-like thing maybe.