Indeed, it seems just as likely that desktop sales will drop faster than Forrester projects. In her report, Epps points out that desktops still offer more processing power per dollar than laptops—in other words, for the same amount of money, you can get a faster desktop than a laptop. As a result, Epps says, people with "processor-heavy" needs—people who want to edit high-definition video or play a lot of PC games—will keep the desktop market alive over the next few years.
But I suspect Epps might be overstating the attractiveness of very powerful machines. The rise of netbooks and tablets proves that, for many tasks, consumers are OK with sacrificing power in favor of portability. What's more, in the future much of the "power" in our computers will come from the Internet. You probably won't even need to store or edit your music, movies, and other files locally for long—we're getting better wireless network drives and Internet-based storage systems, and soon all your media will reside in a central location (in your house or some far-off server farm) accessible to all your machines. You might even be playing graphically-rich games over the Internet soon, too.
You might be skeptical that cloud-based systems will ever be able to match what a big, powerful computer on your desk can do. Perhaps they won't. But that might not matter. The annals of tech are littered with "better" things that we abandoned in favor of more convenient things. We dropped vinyl in favor of CDs, and then dropped CDs in favor of MP3s, even though each new technology offered lower audio fidelity than the previous one. In this way, the shift to portable machines fits larger social trends. More and more people want to work from home, and more and more businesses are allowing them to do so. Laptops, netbooks, and tablets fit into that lifestyle. Desktops don't.
There are hurdles in the way of the many-small-machines future sketched out in Forrester's numbers. At the moment, our computing lives are too scattered. There is little continuity as you shift from a laptop to a tablet to a smartphone to a desktop; each device has different apps, access to different sets of data, and a different screen configuration. Joshua Topolsky, Engadget's editor, recently called for tech companies to create what he calls the "continuous client," a system that will enable you to leave one device and "pick up your session in exactly the same place on the next device you use," Topolsky wrote. "Your IM, Twitter, web browsing, applications, even your windows (given the availability of such a thing on the corresponding platform) appear just as they did on the previous device."
You can bet someone will figure this out soon. And when that happens, you'll no longer think of any single computer as your "main" computer. They'll all be variations on the theme of your personal data. And none of them will be stuck to your desk.