This guy worked for Microsoft for 15 years in the windows OS division. The this video he gives very professional explanation for WHY exactly microsoft is fully dropping support for windows 7/8.1 [*] and pushing for Windows 10.
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[ Mod-edit: [*] Microsoft is not fully dropping support for older Windows versions 7 and 8.1. Rather they are dropping this support on the latest generation of CPU's, such as Intel's Kaby Lake and AMD's Ryzen. This is what Barncules actually says, and what other PC websites have saying for a while now, including Microsoft themselves. In other words, if you have the latest Kaby Lake or Ryzen CPU, you'll need to run Windows 10 if you want Microsoft updates. -- Paul. ]