Windows US desktop marketshare up to 78% as chip deliveries have increased.
Even when considering all devices, mobile included, Windows controls highest percentage, 35%. (Worldwide, Win is on par w/ Android at 35%, as well, but dominates on desktop 79%)
So where’s the “decline”?
See here: http://gs.statcounter.com/os-market-share/desktop/united-states-of-america
It's a question of how big a circle you draw around "what is The Market".
For example, if you consider only traditional "desktop" computers, Windows is unquestionably still in the catbird seat with nearly 80% of -that- market.
But non-business users are abandoning Windows desktops like crazy. As non-business Windows users' desktops die, they are not being replaced by Windows desktops nearly as much as they are replaced by handhelds, and in some cases by other operating systems. Windows 10 saw its biggest uptake in that market because 1) Microsoft gave it away for free for a while, then 2) Microsoft forced it down people's throats for a while, then 3) Microsoft stopped producing Office for anything but Win10, even though Win7 and Win8.1 are still "supported".
Business users of Windows are the remaining desktop market, and it is still large. But it's also still heavily Windows 7, not Windows 10. Business users are not migrating rapidly -- Windows 10 is not popular in business. And when those businesses are eventually forced to abandon Windows 7, a good number will migrate to other platforms.
Meanwhile, in the rapidly growing segment of the computer market -- the cloud -- Linux rules as completely as Windows once ruled the desktop.
That's the "decline". It's not a decline in percentage -- the Sales Dept. pie chart for desktops still appears "good". But the pie is smaller each year.
This is why Microsoft's "Windows Division" was broken up a while ago, and the Windows teams were distributed among other groups. Nadella sees and acted upon that which Ballmer refused to admit -- that Windows as the world's top operating system has been trimmed back to being just another player in a market that has left it behind.
Windows will survive, but it will be for businesses, gamers, and developers. Nearly everyone else is finding other things that suit their needs better.
YMMV. Let's check back in another year or three. :-)