Yeah Win10 isn't sub based. Free upgrades and you get to keep the license for all 7/8 users.
Their main reason? Windows 7 doesn't have the Windows App Store and it would look weird to just give Windows 7 users free Win10 and leave Win8 users in the dust
Cool ! but lets take a moment to remember the fact Apple has done this for years Microsoft copying Apple..tut tut.
Although admittedly it's a bit different. OS X is nothing to Apple w/r to revenue. They probably made it free because it's easier to do that than charging for it. Whereas for Microsoft it is one of their major revenue generators. It's kind of like Apple saying here, have a free iPod.
Errrr so if we want to say that... Linux distros have been doing it since their inception. Everyone just copies *nix. Even the app stores! First Apple, now Microsoft.