Third, aren't Windows sales dipping across the board, anyway, in favor of more mobile devices? That seems like the biggest threat to WinTel, not Chromebooks.
Computer sales in general are dipping across the board, because there's less reason to consider upgrading. Unless you count cell phone/tablet, many if not most of the people reading this probably haven't bought a new primary computing device in years. Heck, I'm typing this on a 3 year old laptop that is still running as well as it was the day I bought it. I have absolutely no reason to consider upgrading it until I start seeing hardware failures, and that could be another few years.
10 years ago, each new generation brought huge improvements in overall user experience/speed. Today, they're incremental at best, and most of the improvements that are being seen in the desktop/laptop markets are to do with power consumption, rather than actual speed improvements. Sure, buying a laptop which will run for 8 hours is better than that 3 year old laptop whose battery lasts 2.5 hours, is it *enough* of an upgrade to make it worth buying a new one? For most of us, no. Case in point: I'm using an inverter that I bought 6 years ago, rather than buying a new laptop with a longer-lasting battery right now (cellular data, too... in the back seat of a car that's travelling 100km/h through the countryside). Sure it's one more gadget to carry, it's still a lot cheaper than a new laptop which wouldn't give me any other improvement.