June 2015 marks the last month that Windows 8 is the flagship OS to buy from Microsoft as Windows 10 will launch on the 29th of July. Seeing that Microsoft is doing everything it can to convince users to upgrade to Windows 10, including putting an icon in their system tray, Windows 8's peak market share has likely occurred in the month of June at 16.02%.
Microsoft is offering Windows 8 users a free upgrade to Windows 10 and considering that the company will pre-load the OS before the release date, on July 29th, millions of machines around the world will be moving to the OS and away from Windows 8 and Windows 7. Microsoft has boasted that they are targeting one billion devices running Windows 10 in the next three years, which means that Windows 8 will quickly vanish from the marketplace.
Based on conversations with enterprise users, the majority of corporations opted to not install Windows 8 and have stuck with Windows 7. Seeing as it's corporations who are usually the slowest to upgrade, this means that the bulk of Windows 8 installs are on consumer machines. Knowing this, the wind-down of Windows 8 usage will likely occur faster than, say, Windows XP or Windows 7.
Also notable this month is that Windows 8 has finally surpassed that of Windows XP. The decades old OS has stuck around despite official support ending for the OS, which means the only way to get patches for the platform is to pay Microsoft a significant amount of money.
Windows 7 is still the most widely used OS by a significant margin and Microsoft does fear that it will have a tough time trying to move these users to the new OS despite the free upgrade offer. Many draw parallels to Windows 7's popularity to that of Windows XP, meaning that this OS is going to be around for a very long time.
It will be curious to see how long it takes for Windows 10 to climb the ranks of the market share ladder, the OS is already reported to be on 0.16% of desktop PCs around the world. With a free upgrade, pre-loading of the OS on machines, new hardware and an aggressive advertising campaign, it shouldn't take too long for Microsoft to reach its billion device goal.