Nobody even knows how many Linux desktops exist, because there are no sales figures to count.
Sales figures do not equate to operating desktops, as anyone who has wiped a pre-installed OS to install something else knows.
Since many desktops have Internet access, the number of operating Linux desktops can be inferred from user agent information provided to Web sites. On my own site, with about 1.6 million unique visitors per year, Linux was reported as the OS of exactly 0.9% of visitors in 2010. Windows represented 84.62% of visitors, and Mac (all versions) represented 11.2% of visitors. About 0.000334% of visitors were still running OS/2, which was a favored underdog OS until its fans (if they were old enough to remember OS/2) switched largely to Linux.
I have multiple Linux desktop machines at work and we have a Linux desktop at home, a Linux laptop, a Linux server and a Linux netbook... I doubt any of them are reporting statistics to anyone.
And I doubt that your arrangement is even remotely representative of the norm. Most people have just one computer, if they have any computer at all, and it typically runs under Windows. People running Linux are often running multiple machines. People running Windows typically have just one machine. I have the impression that Mac users may be slightly more likely than Windows users to have multiple computers (all Apple products), because Mac users are more likely to be fans rather than simple users.
And the desktop market itself has peaked; there'll be desktop computers for a long time to come, but the big growth is likely to be in various kinds of mobile hardware.
I agree. One of the unspoken realities of personal computing is that PCs are designed to assist with intellectual tasks … and there are a great many people who never engage in intellectual tasks. Desktop computers appeal to people who are somewhat more intelligent than average, just as home gym equipment appeals to people who are in somewhat better physical condition than average. This is why computers are gradually converging towards entertainment devices (have you noticed that almost all monitors sold today have aspect ratios that are useless for anything other than watching TV and movies?).
Most people don't even need an application like Word—because they never write anything. Indeed, in the U.S., about 30% of the population is functionally illiterate and can barely do anything with a computer at all.