However think about the advancements in the RIA space during Windows XP's lifespan and think about what will happen during the lifespan of Windows Vista. In 6-7 years is it really that hard to imagine nearly all of your applications deployed as web apps? I'm not saying that the operating system is dead, but I am saying that you won't need it to do everything it does today and more importantly, Microsoft won't be able to charge $200-$400 for it.
In many ways, the "move" to web apps could reopen competition in the OS space for the first time since the 1980s. What starts to differentiate Microsoft from other companies if a lot of what you do is on the web and can be deployed nearly anywhere? Not much. The key is the adoption of web apps by the mainstream. That's where the important of experience comes in, and that's also another blog post entirely, but this means big things for Rich Internet Applications.