Notion of one device is going to be where it is going to be in the near future. In the near future due to the fact that people are going to want less and less items. Right now we have too many devices to use for everything and I don't think vision is going to go anywhere.
When looking at how the world is operating now, some things you will just never know or see, is the future one device, well that depends on a lot, what future are we talking about, Future of technology taking over the world, than if so, if technology taking over is allowed than what happens in the mist of an technical failure?
It is useful to have a big device you can use at home and a smaller one you can carry around on the go. When you're not on the go it's more convenient to use the one with the bigger screen.
And then there are degrees that are in between. Television/PC>Laptop>Phone
No, I do not believe the future will be "one device". I think there will be many attempts by different companies to produce one device that will take the place of our laptops, smart phones and cameras, but I don't see it working for everyone. People's tastes and habits vary so much that it would be virtually impossible to please everyone.
I don't see a cell phone CPU ever doing half the things I do on my
desktop. As it is now, even my overclocked extreme edition Intel CPU is
slow at rendering large amounts of 1080p video. With 4k queued up to hit
mainstream, the CPU is going to be taxed beyond belief. Only those who
have render farms will be able to deliver 4k in a timely fashion.