Well technically, Chrome OS is an OS, but I see what you mean. It's totally purpose-built for browser-based tasks: web, e-mail, cloud apps, and so on. Personally, I doubt that Chromebooks will be a hit. They strike me as being designed for people who don't know much about computers, who don't really want to know much about computers. I know there's a market for that, but I was under the impression that that market was gradually shrinking.
I expect the Windows user base to grow and the Mac user base to shrink. Macs no longer have an edge in terms of processing power. Their only advantage is that they're simpler to use and less prone to viruses, and that's what makes them appealing to creative people who aren't technical-oriented. But as I said before, I think that people are becoming more techno-literate over time, so the need for "easy" computers should be decreasing. If Microsoft could improve the design of their OS, make it a little easier to maintain, and make it virus-resistant, then Macs would disappear completely... which isn't likely to happen any time soon, especially in light of the purple abomination that is Windows 8, but it doesn't matter. People are learning more about computers, and they want the freedom that comes with using the most popular (i.e. most-developed-for) operating system, which is Windows.
Linux is the underdog that will never leave the doghouse. It will always exist, but it will never reach its full potential as a "free OS", simply because of the fact that it's hard to get into. It's kind of a contradiction: if you use Linux, then you probably have some kind of technical job, which means that you can probably afford to pay for an OS. It will always have its uses, but it'll never be fully appreciated.
