1. Do you think every game available for Steam OS will also be available for Windows? ...uh, yes, duh. Steam OS is based on Linux. Have you ever heard of a developer coding a game to run in linux but not in windows? Part of the point of Steam OS is to promote Linux as an option for gamers, but it's just that - an option.
2. Do you think developers who were/are reluctant to make PC games will be more or less likely to make PC games if Steam Boxes take off? If they don't want to make games for a customizable computer because they're too lazy to code for more than one set of hardware or believe the BS that all pc gamers are pirates (because nobody pirates games for consoles, right)? ...Then I doubt anything would change.
3. If they do make games for PC and Steam OS, what makes you think those games will be better ported to the other OS than those made for Xbox or PS4? The problems are usually connected to controls/UI and limitations of consoles. ...what in the world are you even talking about? Usually a game is made for a console and then ported to windows. Steam OS has nothing to do with that; it's simply trying to provide a media center / console-esque experience above and beyond what a cheap plastic box can do.
4. What makes Steam Box a PC? That it runs a flavor of Linux? So does Android. How about the fact that it's made from PC parts, that you can change the parts, install Steam OS to any computer, and have it installed alongside other operating systems?
5. What's wrong with booting multiple OSes are those 30 seconds. I'm playing Hearts of Iron or making a mission in Arma3 on one display, while a video is playing on YouTube on another display. I get an email so I pause the game and read the email, then do something about the email, then just continue the game. Switching between games and other things takes half a second on Windows, but would take minutes (save/exit game + shut down + BIOS checks etc + boot) with dual booting. I got an SSD years ago because I don't want to wait for five seconds while a web browser loads, I'm certainly not going to wait 45 seconds to switch between OSes so I can do a few minutes of work. Oh dear, here goes. First... who pauses a shooter to answer an email? Second, have you ever tried dual-booting before? I can switch from one OS to another in quite less than thirty seconds. Third, perhaps you should consider getting your work done first instead of waiting to do it for a few minutes at a time while your games are paused.