Look at the major PC vendors in the 1970s, then look at the same industry today. You'll find only one name common to both periods: Apple. The company's competitors from the early days of the PC industry have all either gone out of business, been absorbed into other companies, or shifted focus to other industries. Even once-mighty IBM got out of the business of selling PCs to consumers in the mid-2000s.
Not only is Apple by far the most long-lived company in the PC industry -- almost 36 straight years selling computers to everyday people, compared to 24 for IBM and 20 for Compaq -- Apple is also the most profitable PC vendor by a wide margin. Apple has bucked the overall industry trend in terms of both its longevity and profitability; its competitors from the early years have long since flamed out, while its current competitors are, with few exceptions, struggling to stay relevant.
Horace Dediu of Asymco has graphed both Apple's longevity and the number of units it's shipped year over year compared to its competitors, and as usual his graphs provide a very instructive view of Apple's performance (which I'd encourage you to check out for yourself). According to Dediu's graphs, Apple is the only current PC/device vendor shipping more than 100 million units per year; no one else is even close to shipping that much out, and thanks to Apple's tight supply controls, units shipped generally translates very closely to units sold.
When the graph gets adjusted for longevity, that's where the real craziness of Apple's performance becomes clear. Other historical PC vendors like IBM and Compaq enjoyed initial exponential sales growth that eventually tapered off, then ceased altogether. If Apple behaved like the rest of the PC industry, the same thing might have happened to it; Apple's performance looks eerily parallel to that of IBM for most of its life, but the performance takes way off again in the mid-2000s for some reason. Rather than showing signs of tapering off, it keeps increasing exponentially.
Philip Elmer-Dewitt of Fortune analyzed Dediu's findings and says "it puts the lie to Wall Street's consensus view about Apple's future growth." Based on historical performance of other PC vendors, Wall Street analysts keep expecting Apple's performance to taper off and reach a more steady state -- still positive growth in sales and profits, but more of a linear growth than the huge gains of the past few years.
Looking at the big picture, though, that view isn't supported at all. Apple's performance looks nothing like that of its competitors when you look at its entire history; if Apple had behaved as it "should" according to what's happened to its competitors, then it should have gone out of business in the mid-90s as we all feared it would. Instead, Apple keeps redirecting its focus rather than staying the course, and its performance over the past five years clearly reflects that.
If Apple had never introduced the iPhone and iPad, the company's growth almost certainly would have slowed down years ago; man cannot live on Macs and iPods alone. Yet though Apple is by far the longest-lived of the PC vendors, it owes that longevity to a willingness to adapt. Both in terms of the products it sells and its overall focus, the Apple of 2012 is a very different company than the Apple of 1976, and the fact that the company has developed itself into a swiftly moving target means it's unlikely Apple will suffer the fate of its historical competitors anytime soon.