I think the real threat to PC is that most people are buying a laptop/tablet/smartphone for their office work/email/internet browsing and they are getting consoles to play games. A modern laptop consumes less power, is portable, has decent quality screen and is fast enough for just about anything (and feels just as snappy with an SSD). Unfortunately, I think a lot of people simply don't know or don't want to learn how to assemble a PC.
- More and more jobs involve travelling as a result of globalization and companies selling products/services across the world now (a desktop isn't suitable for this type of lifestyle),
- The 9-5 / 40 hour work-week is quickly disappearing. Many companies want their employees to work from home (after the regular 9-5) (i.e., they give you a laptop so you can't say that it's too heavy to bring home!), so on a corporate level there is a huge incentive to shift away from desktops (unless you need all the power you can get),
- People want to have constant access to information 24/7, no matter where they are (a desktop can't provide that),
- A lot of main disadvantages of laptops are in the past (battery life is now 7-9 hours long, performance is plenty fast, laptops are thin and light, and screen resolution is much improved with 1080P options),
- It's trendy to be seen with a MacBook Air but hardly anyone can see your $5,000 desktop PC. In other words, I hardly imagine that the new generation which is growing up today finds a desktop "sexy" or "desirable" or "fashionable",
- Also, the "green movement" isn't helping desktops which take up more space and consume a lot more power than a laptop/tablet/smartphone.
I can see how for the majority of the new generation of consumers it simply doesn't make sense to buy a desktop anymore. Although I believe there is still some growth on the desktop side, even if very small. When we start to see consecutive years of declining desktop shipments, then we can start worrying. I don't think we are there just yet.