Though I think it will be possible, technologically speaking, before very long, I don't think it's a likely direction for the big console manufacturers. I reckon that integration between handheld and console devices will continue to increase and will become ever more seamless, but ultimately, no manufacturer is going to release one box when they know they can release two, and still have a good chance of shifting large quantities of both.
There's just too much money to be made with manufacturers like Sony and Nintendo having separate handheld and home consoles for them to do away with one of them, even if technologically, the home console becomes surplus to requirements. Then there's the way in which home consoles are set to evolve to think about. Who's to say that home consoles 20 years from now won't necessitate a control set up that's impossible to make portable? We could be talking about VR headsets, interactive life-size holograms or any number of other dramatic changes that wouldn't work on handheld.
I have a good compromise though. If Nintendo, Sony or Microsoft were to release a handheld console that had wireless access to the Virtual Console, Xbox Live Arcade and PSN services respectively, that was compatible with every title on each of those respective platforms and was capable of wirelessly transferring your save games between each platform on the fly so you could go between each console and never have to replay a section you had already completed, I would definitely buy it.
Like a tramp in the night, I'm begging for your love - Samantha Fox