The whole analogy thing in the OP is pretty odd. Of course we can compare Ipods to home stereo systems, it'd be pretty interesting to do so. I don't see what's wrong with comparing Portable DVD players to non portable dvd players. It's not really very informative to know that a certain number of people of Portable dvd players were sold, the number needs context. As for comparing tablet sales to PCs, people make that comparison all the time.
http://thinkgeek.com.au/mobile/will-tablets-really-cannibalize-personal-computers/
http://ipadlatest.com/news/Consumer/20110126/19088332/Tablet-vs-PC-Sales-In-2011-Controversial-Message.htm
http://www.pcmag.com/article2/0,2817,2383096,00.asp - PC Sales Slump While Tablets 'Usurp' Market,
http://www.theage.com.au/digital-life/computers/tablet-hype-gutting-pc-sales-20110414-1df4o.html
Guess I must be an alcoholic, as are the majority of mainstream journalists.
