While it is true that most mainstream OSs' focus on security has heightened in the past few years, some had an eye on it the whole time. Some were always meant to be multi-user from the git-go. Some have been built on a base that lends itself to security. Windows is not one of those. That's not to say that any others have it perfect, but the road map you describe is the one of Windows, not everyone.

There are some things that Windows has done right. As has been well documented in threads here recently, they do have a more robust set of access controls than the traditional *nix model (at least in Pro). But, there's a reason that Longhorn has been delayed so much. And there's a reason why xp sp2 broke so many things for so many people. They have tried to introduce security into an insecure system, and to do it, they had to change too many things. Starting from scratch is probably the best thing they can do, but then how pissed are people going to be when they can't run their old apps on it? How many developers are going to have to do a serious rewrite of their code?

My thing is, don't say that "all operating systems are the same"; they're not. There is a difference. And Window's performance record, with regards to security, has been shoddy. More shoddy than many other systems, and it's not because there are more of them out there.