Many of the workstation-synth keyboards are nearly computers already. It really wouldn't take much to put a full MB and a CD/DVD drive in one. But what we really need is a DAW built like a mixing board. With it's own brain-processor, all the analog and digital inputs /outputs, Advanced USB connectivity, multiple headphone mix-output capabilities, main outs, control room outs, sub outs, effects sends and returns, real time faders and meters And that's a short list.
While most of this is acheivable via software routing, processor scheduling, etc., there is still a short fall between a real time high end 8 buss mixer and a computer based DAW. Many of the new control surfaces, the VS 700 being one, have begun to address this concept on smaller scales but there is still a gap in real operational design at the DAW level of control and routing of seperate signals and the recombining of those signals after editing, processing and alteration.
I wouldn't say it will never happen, remember Open Labs? To say that it won't be main stream profitable is to say that consumer level audio will never come up to professional standards. Perhaps professional standards have sunk to consumer levels? Or maybe the transition is somewhere in between.
But it's still somewhat of a mismatched bunch of technology trieing to become something everyone wants to buy.