Microsoft undoubtedly did similar calculations before it boasted about the one billion target.
But other experts questioned the goal, if not the number itself then the rationale behind it.
"Windows 10 is a huge step forward," said Jan Dawson, chief analyst with Jackdaw Research. "A very large proportion of the Windows base will be on Windows 10, and I don't doubt that it can make the goal. But the whole billion is fragmented across all these different devices, PCs, tablets, phones. Yet phones are where are all the developers want to be. That's where people are spending money and time, on smartphone apps."
Dawson's point was that while there may be a billion Windows 10 devices in two or three years, the vast bulk of them would be PCs, not mobile devices that developers are most interested in, where apps actually sell.
Microsoft's second strategic pillar for its developer pitch was its universal app model and the new tools, announced this week, that theoretically make it easier to port Android and iOS apps to Windows. But Dawson wasn't buying those arguments either.
"[One] problem is that porting apps often results in a lowest common denominator approach, in which the very people most likely to be attracted to a lower-cost porting method are those least likely to be willing to put the time and effort into making the experience really shine on each platform," Dawson wrote in an expansive piece on his blog.
So is the one billion goal really reachable?
Microsoft has touted numbers like this before, notably in 2012 when then-CEO Steve Ballmer seemed to say that the impending Windows 8 would be on 500 million devices within its first year. Although Microsoft later asserted that Ballmer was misquoted, he and the company continued to argue that upgrades to Windows 8 would create a lucrative audience for app developers.
In September 2012, Ballmer stepped in front of developers to make the Windows 8 case. "There will be customers coming and looking for apps. That I can assure you," Ballmer said at the pep rally-like event. "It's going to create a heck of a lot of opportunity for folks in this room to make millions."
Everyone knows how that turned out.
With enterprise adoption of Windows 10 expected to be lukewarm for the first few years after its release, Microsoft's goal will rely on consumer PCs, both bought new and upgraded, and improved sales of mobile devices.
There's some precedent that can be used to gauge the upgrade tempo for Windows 10, which Gillen of IDC predicted would make up more than a third of his quick forecast. Computerworld has used the upgrade rate of Windows 8.1 — like Windows 10, also handed to customers for free — to estimate that up to 358 million existing PCs could be running Windows 10 with a year of the new OS's launch.
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