Think 2013 has been bad for Microsoft? Just wait until 2015.

With plummeting PC sales and Windows 8's struggles, 2013 is proving to be a year that Microsoft would like to forget. But there's evidence that it's nothing more than a preview of even worse things to come. By 2015  Windows will likely fall to the world's number three operating system.

How bad is 2013 turning out for Microsoft? Sales of traditional PCs will fall by 10.6% for the year, finds a recent Gartner report. Sales of Windows RT Surface tablets have been dismal, so bad that Microsoft may be sitting on unsold inventory of 4 million. Next year doesn't look much better for Microsoft, with traditional PC sales falling another 7.3%, Gartner found.

The trend overall is against Microsoft, with mobile devices including smartphones and tablets taking up an increasing share of the worldwide computer market. That's Microsoft's weakest link. Making matters worse for Microsoft is that consumer-oriented mobile devices will begin to take over the enterprise as well. The Gartner report notes:

"With enterprises' growing acceptance of bring your own device (BYOD), there is an increase in consumer-owned devices in the computing world. Gartner forecasts that computing devices bought by consumers will grow from 65 percent in 2013 to 72 percent in 2017. This signifies the growing importance of designing for the consumer inside the enterprise."

What does all this mean? By 2015, Windows will fall behind Apple, says Gartner. Today, Windows is the number two operating system in the world, with a 15.6% market share to Android's 22.8% market share, and Apple's 9.6%. (Apple's share includes both iOS and Mac OS X. By 2014, Android will grow to 42.3%, Microsoft slip to 15.1%, and Apple will grow to 14.2%.

By 2015, Apple's overall market share will exceed Microsoft's. So says Carolina Milanesi, Gartner research vice president. She says:

"Apple is already leading the consumer market. They are selling more than Windows today.... Microsoft remains relevant thanks to enterprise and professional users."

But as the consumer market grows, and as consumer devices increasingly invades the enterprise, Microsoft will lose that edge. That's why Microsoft will slip to number 3.

By 2015, of course, Microsoft will release one more new version of Windows. If it's a killer operating system, that could possibly fend off Apple. But given Microsoft's recent history, that looks doubtful. By 2015, expect Microsoft to slip behind Apple as well as Android.

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