Why Microsoft Windows 10 is the last OS from Microsoft?

It's been some time since we are hearing that Microsoft said that Windows 10 is the last OS from Microsoft. So me and some of my friends try to find out that why it's the last OS from Microsoft so as much as I found out is that Windows 10 is not even a new Windows Because it is a mixture of windows 7 and Windows 8 or 8.1 because it got all the features of windows 8.1 except the only annoying one in which the users get the full size start menu instead of the small start menu. Which is great because in Windows 10 the users get the small start menu with the live tiles.
         But since Microsoft has decided to make it last longer or the last Windows version they need to give some thing different then other OS developers. For example in Android Google is rolling out new versions of android every year because of which the developers and the users are suffering because they have to buy new devices every year to stay up to date. That is were Microsoft is hitting. Microsoft is upgrading every device running Windows 8 or 8.1 and Windows 7 (running in computers) in to Windows 10 and its updates. which means that without even changing the divorces the users will be running the latest version of the Windows. Which is a good marketing strategy to attract new users.