Industry analyst Horace Dediu, writing this week on his Asymco blog, crunched the ratio of Microsoft-based machines sold versus Apple ones over the years. Following the introduction of the Mac in 1984, he noted, the ratio of MS-DOS machines to Macs was roughly six to one, or about 2 million PCs to 372,000 Macs by 1995.

Not Just 'Arithmetic'

When Windows 95 was released in 1995, it led to an even greater boom in PC sales. By 2004, the ratio was a crushing 56 to 1, with about 182 million PCs sold versus about 3.25 million Macs.

The ratio between platforms "is not just an exercise in arithmetic," Dediu said. He pointed out that it's also a "measure of leverage," in that "ecosystems become self-perpetuating and there is a tendency toward monopoly."

In other words, he wrote, "the stronger you are, the stronger you get."

Then, by 2004, the MacBook began to chop away at Windows machines' dominance, and the multiple of Windows units to Mac units fell to about 20.

Now, in an age when the center of computing has arguably moved to mobile devices, Apple has a huge advantage in tablets and smartphones. Windows Phone devices are still trying to gain a major foothold among smartphones, and Microsoft has retooled its OS into touch-centric Windows 8, hoping to secure a beachhead among tablets.

Parity Soon?

As a result, if one considers Apple devices using both Apple platforms -- iOS and OS X -- versus all Windows device platforms, Dediu said the current multiple of difference is less than 2.

He estimates that one can expect Apple devices to reach a parity with Windows devices within the next couple of years. At a time when Microsoft is struggling to avoid irrelevance in smartphones and tablets, that could be a significant blow to its central strength, the market position of the Windows operating system.

Key to this transformation are the indications that tablets and smartphones, plus their growing universe of peripherals, are overtaking laptops and desktops as users' main computing devices -- or, at the very least, becoming their equals.

Surveys and field data are supporting this hypothesis, even if smartphones are not included in the mix. For instance, earlier this year pollster Poll Position surveyed 1,100 people in the U.S., and found that nearly half thought that tablets would, sooner or later, replace laptops.

And, on Thursday, NPD DisplaySearch revised an earlier study comparing future growth of tablets versus mobile PC units. While that earlier report in January showed tablets reaching about 47 percent of all mobile PC shipments by 2017, NPD now predicts that, of the 809 million mobile PCs sold in 2017, 51 percent will be tablets.

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