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.
After XP, though, the work in the core OS was basically done
There were a lot of big things happening in computer hardware: migration to 64-bit instruction sets and memory addressing, multicore processors, the rise of the GPU. The security paradigm also shifted to less trust between programs, with a lot of implementation details on encryption and permissions.
So I'd argue that Windows has some pretty different things going on under the hood from what it was 20 years ago.