What is Windows? It's that thing, in the background, that lets you run all the applications you want to run.
But what if all your applications were written in Javascript and HTML, and had URLs instead of .exe files? With all the extra power built into Chrome, they could still write to file systems, store data, work offline, do sophisticated graphics and parallel processing -- these are not hypotheticals, this is what Chrome can do right now. We're just beginning to scratch the surface of what an app written from Chrome can do.
Chrome can be an OS, for some value of "OS".