POST-PC == POST-TRUCK ... but look at how well trucks still sell...
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When Jobs said, this is the post-PC era, he was deliberately referring back to the "post truck" era of the early to mid 20th century when more and more people were living in urban vs. farm areas.
In other words, as more people become consumers than producers, the need for a truck lessened, and people began buying more vehicles for commuting, fun and show.
The problem with this analogy, of course, is that even in the post-truck era, a truck has remained the single most popular selling vehicle model in the US.
Moreover, all over the world, people have found that combined fun/work vehicles (e.g. SUVs) are far more useful in the general case. This doesn't stop younger people from buying cheap cars, and it doesn't stop richer people from buying sports cars, but family vans and SUVs just make more practical sense.
So, taking the analogy to its logical conclusion, it would seem that an "SUV" approach to computers (perhaps like the Surface) will become common, even while "car" tablets remain popular as well.
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"I'm trying to think of a good analogy. When we were an agrarian nation, all cars were trucks. But as people moved more towards urban centers, people started to get into cars.
"I think PCs are going to be like trucks. Less people will need them. And this transformation is going to make some people uneasy... because the PC has taken us a long way. They were amazing. But it changes. Vested interests are going to change. And, I think we've embarked on that change.
"Is it the iPad? Who knows? Will it be next year or five years? ... We like to talk about the post-PC era, but when it really starts to happen, it's uncomfortable." - Steve Jobs, 2010