This, and we're almost there. I like having a tower with parts I chose and installed myself, but I'd totally use my phone(loving my new S5 btw) in a dock to turn it into a laptop or tower. Current phones are powerful enough to do what I want in a portable computer. They just need to be dockable. That Ubuntu phone that was kickstarted would have been perfect, but a bit spendy.
For /average/ person, the power's already there. For the high end pro, it'll be awhile longer, but not that long. It's in the foreseeable future. The biggest restrictions are cost and size at this point.