I think Jeff Clarke is looking in the wrong direction for the "middle class" market given that a LOT of people use their phones for just about everything.
In the next 20 years, I don't see the middle class needing to buy a new computer because of sites like craigslist and the fact that Microsoft is moving to tablets, phones, and PC unification into one OS from what Windows 8 has shown us so far if MS keeps moving down this path that is.
Everyone will need their own phone, while with PCs a family can share one. Odds are mom and dad already have one at this point given how digital we are now. I see the PC market stagnating sadly, at least until something new and revolutionary comes out in those 20 years that makes a current top of the line PC look like a TI-82.