Are declines in PC sales in any way surprising? Frost the past decade and a half a larger and larger portion of PC sales have been laptops. Schools from junior high through college practically (or actually) demand them. The proliferation of WiFi means just about anywhere with a roof is going to offer some internet connectivity. Besides ubiquitous internet access laptops have gotten way more consumer friendly by getting ever cheaper and lighter. For just about everyone a laptop is the form factor to buy.
For most of the past 15-16 years laptops were getting faster CPUs or way better GPUs every two years or so. Battery life didn't improve much but at least the machines got more powerful. The past 5-6 years though the landscape has changed. Fewer laptops ship with discrete GPUs as Intel's have increased in capability. Even low end laptops have SSDs and 8+ gigabytes of RAM. The usable lifespan of laptops has increased significantly. Even a change from an average of two to three years means fewer sales for manufacturers. There's a non-trivial portion of the laptop market that's seeing a replacement cycle of over three years.
In addition the sort of things people needed a laptop for ten years ago can be as effectively or more effectively done on a phone or tablet. Android and iOS tablets beat the shit out of Windows tablets and 2-in-1s because hey aren't saddled with a heavyweight OS that honestly is not designed to turn on and go and then back off just as easily.
Billions of smartphones and many millions of tablets have definitely sucked the oxygen out of the room for traditional PCs. With PCs not "needing" more regular upgrades is choking the PC industry. The PC market is saturated and is not likely to grow again. Emerging markets are not a savior because they don't have the same infrastructure as developed markets. They aren't going through a dial-up landline internet connected to a beige box phase. They're going right to smartphones, tablets, and other highly mobile devices that fit better in their infrastructure.