Disclaimer: I have never seen anybody use a tablet with a keyboard. I don't own one nor do I expect to get one in the foreseeable future.
The (retina) iPad's screen is easily more than 3x as good as that on any netbook I've seen (and in fact better than any PC laptop), even a good phone has better resolution, color and viewing angles. If they sold them as computer monitors I would buy one. If we weren't stuck in the dark ages of fullHD TN panels I'm sure there would be more demand for computing power on desktops, too, but what good will it all do if your photos, websites and games won't look any better? If you can't even have a whole page of a book shown on-screen at once without the text being rendered at such low res as to make it genuinely difficult to read? There are diminishing returns on computing power when your user interface devices are the limiting factor.
I think part of the success of tablets has to do with how advances on traditional PCs have been made too difficult. Windows isn't resolution independent. Netbooks were intentionally marginalized. Outside linux-style package managers you still had to hunt your software down one-by-one from all over the internet until Apple's App Store. The trends for easier and more mobile computers have been apparent for decades. Perhaps "general use" computers aren't all that general use after all if they can't keep up?
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