>> people are not upgrading to Windows 10
> it's already on 700 million PCs
Since Windows 10 was released and forced onto every new PC made by OEMs there have been nearly 700 new PCs sold. There are no figures for how many of those are still running Windows 10 (granted most of them will be).
The original poster referred to 'upgrading'. There are still more Windows 7 PCs than W8 + W10 in spite of all new PCs for the last few years having later versions.
https://netmarketshare.com/operating-system-market-share.aspx?options=%7B%22filter%22%3A%7B%22%24and%22%3A%5B%7B%22deviceType%22%3A%7B%22%24in%22%3A%5B%22Desktop%2Flaptop%22%5D%7D%7D%5D%7D%2C%22dateLabel%22%3A%22Trend%22%2C%22attributes%22%3A%22share%22%2C%22group%22%3A%22platformVersion%22%2C%22sort%22%3A%7B%22share%22%3A-1%7D%2C%22id%22%3A%22platformsDesktopVersions%22%2C%22dateInterval%22%3A%22Monthly%22%2C%22dateStart%22%3A%222017-12%22%2C%22dateEnd%22%3A%222018-11%22%2C%22segments%22%3A%22-1000%22%7D
Biting the hand that feeds IT
