From the very beginning, it always had particular features which were designed to make things worse for the users for some business reason for Microsoft. After XP, though, the work in the core OS was basically done - it wasn’t slow or lacking important features or unstable (relatively speaking, at least), and so the only changes being made to it from then on were adding crappiness to it for some reason related to business priorities or just simple stupidity. And so, it entered its slide.