It's good if they're chucking out the old codebase, simply because of the law of diminishing returns.
What I'd hope - and I'm not sure if this is the way some other O/S' are structured - is that they (or someone) will develop a twofold layered O/S. One layer would be the hadware, etc, base stuff; nothing to do with security, fancy UIs, etc, but just the raw process facilitation stuff. And then on top of it another layer that handle stuff like security restrictions, fancy windows, etc.
I'm not sure exactly how you'd deliniate that, to be honest. But it strikes me that windows is - and has - moving too far from the simple responsibilities of an O/S and implementing features that are really either restrictive to applications or things you'd expect in applications. Anti-virus and anti-spyware things are, IMO, applications and not O/S responsibilities (for example).