Windows 10 as released in June 2015 wasn't complete. They were probably trying to rush it out for back to school or some other milestone date because their original launch period was fall. They're adding missing features and fixing broken features that shipped with RTM code. Windows is never "done" - they always refine shipped features, add security updates, and add new features as the tech market develops over time, but 10 didn't even meet the "done" of past releases. MS would figure out what features to add to the OS and work on it until all those features were in; maybe not perfect but, all there. Win 10 didn't even match that, where the missing features are being released at the end of the year in a "Threshold 2" update. And those TH2 features will probably be half-baked too, requiring more stability updates. But that, along with the "you take all the updates because we said so and no, we won't give you patch notes," is part of Windows as a Service. Like modern video games, you get a mostly functional product on disc that requires a lot of patching on day 1 and maybe a multi-player mode to be added later.