Also, you're not a soothsayer. SteamOS is much more game-oriented than Windows, and it's fully possible it'll eclipse Windows in use by gamers.
The only way it would eclipse Windows is if developers start creating games for it, their seems to be a fallacy amongst the majority that if Valve makes it they will come, variants of Linux and it's ilk that can run games have been about for a good amount of time and their are still hardly any major developers using it or for that matter dual releasing on Windows alongside Linux.
MS don't charge for developers to create games on their OS, the tools and development kits for DX are out there, free to download and use and more importantly they are well developed, so given that the vast majority of the titles that use Steam will still require a Windows powered PC running to work, alongside the fact that which ever way you slice it the OS Steam will be releasing will still be new and subject to all the issues and development that goes with a new OS combined with the fact that after ten years Steam itself is still one of the slowest, bloat filled nonsense gaming clients that you can get, where exactly in all this is the proven skill set that Valve have got the chops to take on the Windows gaming market yet alone making it so big that they will force developers away from Windows to their own OS?
The real question is now that Valve have announced this, what are the reactions from the people who actually develop games? Have any big names come forward and expressed even the slightest interest? I haven't found anything yet.