Anecdotal: the PC and laptop I bought around the time tablets started coming out are starting to show their age. So now I'm in the market for their replacements. Could a factor be the fact that the age of the tablet market currently equals the average lifetime of a PC?
Good point. Here's a theory: laptops were running out of time and people had to opt for a new laptop or a tablet (which, at the time, had the novelty factor) and most of them decided to try a tablet. Tablets did not replace the old laptops so (some) people have since bought a new laptop. This could explain why laptop sales were down during the first stages of tablets' market penetration.
That doesn't make sense. Not unless the introduction of the tablet caused people to buy PCs and laptops. There might be some dynamics between the two, but it doesn't relate to the age of the tablet being equal to the average lifetime of a PC.