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In shorter timeframes you might have a point, but as integration increases that's not really how tech evolves - it does tend to give much more for much less, over generations.
Electronic calculators or even pendrives were a big deal not a long time ago - and they can be quite easily found as freebies now, and certainly for the price of a lunch. DAPs similarly (quite expensive, unwieldy and limited a decade ago - but now few lunches give a very small and capable unit). Present sub-$100 no-contract smarpthones are already much nicer than an order of magnitude more expensive (then) decade-old ones - and still only revving up their economies scale (some of their characteristics should ease further lowering of prices - for one, much simpler mechanically than "classic" mobile phones, basically just a screen)
BTW, $20 is not the true price of OSX - it's basically subsidised by hardware sales. Also, Linux is used by most smartphones... (and not a long time from now, most likely the same for tablets)