Rest assured, I'm pretty confident that won't be the case. While web apps certainly have gained their fair share of popularity, some things still need to be localized on your machine. Chrome OS is just proof of that since it has really taken off after it had forgone its original goal of being fully web based. Last I checked, it even had Steam working allowing you to play games on YOUR computer. Also, keep in mind if your fears did come true, Microsoft would have to run a cloud instance for every single computer running windows on the planet at the same time. This might just be my optimism and faulty assertion, but I don't think that's something they would want to do. A subscription based OS is likely, though.