Part of the decline is definitely that the PC is a mature market, so fewer new people to sell to, and the tablet is a new market. But I wouldn’t discount a shift in the way people are using computers - much more time on mobile phones and tablet, and less on desktops. Also, lighter cheaper notebooks are much better daily use alternatives to desktops than they used to be. I’d love to see how much traffic here at free republic is now from mobile or tablet devices.
Definitely a big shift hit when the laptop became cheap enough to be the primary personal computer of anybody who feels the need to be mobile. Tablets are a number of generations away from having the raw power to be a primary device, most of the folks I know who can claim to use them as a primary device remote desktop (or equivalent app) from them into “real” computers, so while it’s the device they touch the most it’s not really their primary. The business market, and people like me who feel no need to be mobile in their computing, will keep the desktop alive, though probably never again the primary section of the market, for a long time, just because the larger version of technology is always cheaper than the smaller, and desktops have the space to use the larger parts. There’s always going to be that section of the market who wants to maximize their power to dollar ratio and is willing to sacrifice mobility, especially with huge monitors dirt cheap.