But neither Microsoft's nor Apple's operating systems will hold a candle to Android. In 2017, Android will ship on 1.6 billion devices, or 65% of the total, up from a projected 1.3 billion and 54% this year.
Overall, Gartner's newest forecast was more pessimistic for all device categories than previous estimates this year, in part because -- as it's said before -- of ever-lengthening lifecycles.
"Replacement activity across all types of devices has decreased," said Ranjit Atwal, a Gartner research director, in a statement. "Users are extending the lifetime of their devices, or deciding not to replace their devices at all."
That trend had long been identified in personal computers, then later linked to tablets as well, but growth in smartphone shipments has also been affected. Gartner's July forecast, for example, had pegged smartphone shipments to grow 3.4% in 2015, more than double the latest projection of only 1.4%.
The bulk of Apple's gains against Microsoft in the OS skirmish for second place will be powered primarily by the iPhone, of course, but that demonstrates Microsoft's inability to get Windows on smartphones as much as Apple's continued strong sales.
There's little chance that Microsoft can turn around Windows' future as a mobile OS any time soon, on smartphones in particular: After the Redmond, Wash., company wrote off its Nokia acquisition in July with a massive $7.6 billion charge against earnings, it also retrenched on its device strategy. While Microsoft will ship new flagship Lumia smartphones this year, it has admitted it must downsize its own smartphone hardware efforts.
Gartner also pointed out that shipments of PC-like devices -- Windows' bread and butter -- will continue to contract.
Its September forecast set the combination of traditional PCs and premium "ultra-mobile" devices -- Gartner defined the latter using examples like Apple's MacBook Air notebook and Microsoft's Surface Pro 2-in-1 -- at 291 million shipments for 2015, a 7.3% decline from the year before.
Today's forecast for PCs and premium ultra-mobiles was gloomier than those Gartner made three, six and nine months ago, when the research firm said OEMs would ship 300 million, 306 million and 321 million, respectively. Premium ultra-mobiles, however, will remain the bright spot in an otherwise dim future: By 2017, manufacturers will ship about 78 million of the pricier devices, nearly double the 44 million predicted for this year.
Not until 2017, when businesses begin to replace their current systems, will old-school devices -- and thus old-school Windows -- show some meaningful growth. It's no coincidence that the year-over-year increases for both PC and premium ultra-mobile and Windows will be identical in 2017, at 4.5%.
"Windows 10 adoption among businesses will ramp sharply in 2017, where we expect the PC market to return to a 4% growth," Atwal said.
Microsoft hopes so.
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