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.