I will tell you why MS is pushing so hard for Windows 10.
I will tell you why MS is pushing so hard for Windows 10, since apparently it is not obvious.
Windows 7 is insecure, as is all complex software, and the insecurities are too widely known.
Removing the known insecurities from Windows 7 was difficult and expensive, and what was created in doing so was Windows 10.
Maintaining 4 OSs, adapting them to new hardware, fixing the inevitable vulnerabilities as they are discovered, and so on is much more effort than maintaining one OS.
The big complaint with Windows 8 and to an extent 8.1 was simply that the UI was different from Windows 7. That complaint has been fully addressed, so that customers can switch over and so that MS can save money by not having so many OSes to maintain.
It is true that by giving us a free operating system that is so much more efficient and multi-threading than Windows 7 it will decrease hardware sales -- hardware people shouldn't be happy -- but money saved on hardware is money available for spending on software. Part of the money I saved by not needing to buy a new computer I used to upgrade from Office 2010 to Office 365.