Forecasting Windows market share

Forecasting Windows market share:
Last week Frank X. Shaw, VP of corporate communications at Microsoft stated:

 … most of the people around me were using their iPads exactly as they would a laptop – physical keyboard attached, typing away, connected to a network of some kind, creating a document or tweet or blog or article. In that context, it’s hard to distinguish between a tablet and a notebook or laptop. The form factors are different, but let’s be clear, each is a PC.

Actually this “admission” that iPads are PCs is not something new. Steve Ballmer made the same assertion in 2010 pre-iPad (though calling them slates). Arguably, the notion that tablets are PCs has been dogma at Microsoft for over a decade and Windows running on all form factors has been a strategic guiding principle.
Which is why I’ve always added the tablet data to the PC data to create a picture of the “personal computing” market. And this is what that picture looks like today:

Note how the share of various platforms has evolved over this brief time span:

Seen this way, Windows has now reached 60% market share and it’s likely to dip below 50% during this year. What happens beyond then is harder to imagine. If Windows tablets start growing as fast as the tablet market overall then Windows could stabilize in share. But if Android and iOS tablets follow their phone brethren in growth then it will be far harder for Microsoft to maintain share. But is that cause for concern?
Not necessarily.
The total computing market[1] is likely to expand to over 4 billion users with 1.5 devices per user in the next five years. That expansion implies that 20% share equals more than one billion devices, making such an ecosystem “good enough” for the average developer. It certainly has been good enough for Windows developers to date and they have kept hiring it throughout the new mobile app revolution.
So even if Windows dips to only 20% of the world’s computing market it will still be perfectly “viable” for some time to come.
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1. I define the computing market as the total number of devices which have (a) a CPU (b) a broadband connection (c) a native application execution environment which is open to third party apps. This definition implies the presence of an “ecosystem” which is bound specifically to a platform.