Microsoft has reported the latest milestone for Windows 10, with the operating system now running on 400 million devices.

That’s an impressive start for the now year-old Windows 10, but it also suggests that adoption of Windows 10 has slowed, at least for the time being (see chart below).

The new operating system got a handy early boost largely because Microsoft gave it away for free to consumers with PCs running Windows 7 and Windows 8.1 (and owners of Windows Phone 8.1 smartphones). But with the free upgrade offer over, that boost has gone away.

And while 400 million looks pretty impressive, Microsoft also gave itself – perhaps unwisely – the public target of having the OS running on one billion devices by 2018, only to then had to admit it wouldn’t make it.

And while Microsoft hasn’t given a deadline for when it now expects to hit one billion, that figure still lingers in the mind of many as a mark of the success or failure of Windows 10.

The problem here is that the number of PCs sold every year continues to decline: worldwide PC sales have now dropped for seven consecutive quarters, according to Gartner. And the total number of PC in use is sliding down too: the analyst predicts the total number of PCs in use will decline from 1.485 billion last year to 1.442 billion this year and 1.4 next year.

All of this means Microsoft is trying to hit an ambitious target of one billion devices running Windows 10 in a fast declining market. And most likely Microsoft had been banking on tens – perhaps even hundreds – of millions of Windows smartphones to be running a version of Windows 10 too. But thanks to an expensive change in strategy its smartphone market share has dropped below one percent.

Around 280 million PCs were sold last year, and perhaps a few million less will be sold this year. The majority – but certainly not all – of those are likely to come with Windows 10 on board. That could add somewhere between 150 to 200 million PCs running Windows 10 each year, maybe even more, every year. But it’s also worth remembering that many of those will be replacements for existing PCs – and the size of the total PC market will actually be declining over this period. Other devices like Xbox and Holo Lens and smartphones may help close the gap, but for now it looks like the majority of devices running Windows 10 in the next couple of years will be PCs.

It’s now down to sales of new PCs and business migrations to pick up the slack.

Certainly Microsoft is trying to make it as easy as possible for enterprise customers to make the leap, most recently at this week’s Ignite conference, first by pointing out that Windows 10 security outclasses that of Windows 7, which is where most businesses are sitting quite happily now. And it is also providing more tools to make it easier for businesses to make the upgrade too.

So will enterprises take up the baton now the first wave of consumer upgrades is over? One consequence of the free offer is that it has created more consumer momentum behind a Windows upgrade than before: perhaps it will lead to consumers demanding the same in the office, encouraging the CIO to start the roll out sooner rather than later.

But it’s also likely that the giant operating system migrations of the past are gone forever: over on ZDNet Mark Samuels interviewed some CIOs who now viewed the operating system that their PCs are running as a much lower priority than making sure their applications and data are accessible everywhere.

That means the cloud is increasingly the computing platform of choice and the local operating system, whether it’s iOS, Windows, Linux, Android, Chrome (or even Andromeda if it ever arrives), becomes much less relevant. That means big upgrade projects may be less relevant than before. Microsoft may well still hit its one billion target eventually, but it may be a bigger job than it first imagined.

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