As for the "three years" projection, I'm not sure what the article is saying. Our (mobile) data will be in the cloud within three years? Probably. That Apple will kill Microsoft in three years? Absolutely not. No way. Don't underestimate the power of inertia. Even if Apple manages to entirely kill Microsoft's consumer market share (unlikely) they can survive off enterprise customers just fine. I mean, what do you use on a Mac to compose documents? Word. Are there any others? (I don't have a Mac, so feel free to correct me if I'm mistaken.)
I don't think PCs are going anywhere. People will always need to get serious work done, and larger systems will always be better at that than smaller systems of the same era, if only for the sheer amount of numbers they can crunch. Mobiles are great products with a tremendous amount of utility, but they're complements, not substitutes.