I'm pretty sure, if SteamOS and SteamBox will be successful.... it will not help the Linux gaming (in current understanding -> support the distros) that much. It will just transform "Linux gaming" to "Steam platform gaming" (on Linux), similar to how Android is designed (on top of linux kernel, minus GNU, linux userland stack and distro mess) or MacOS (on BSD kernel). For later one also no one saw a rise in BSD apps or games.
Architectural, SteamOS will take out the wonky distro and unix aspects out of the linux ecosystem, forming a addressable platform. Exactly the parts the linux geek ecosystem is so proud on, unlimited choice and customizability, gone.
If they use the Linux kernel, then any driver updates for Steam OS will be compatible with the Linux kernel, or at least will have a known set of changes that Linus can implement.
Otherwise, I doubt that Steam OS is going to be so much good for Linux as bad for Windows and OSX. OTOH, getting people away from OSX and Windows to see what things are like should be good for people in general, and Steam OS games should be like 95% ported to linux in general, and probably even closer.
While indeed driver support might become better, I don't believe general linux ecosystem compatibility of apps or games will rise, I assume the opposite: once the Steam API is available as stable, addressable platform for Linux, game developers will use it happily and completely drop all (fruitless and demanding) tries to support something else (e.g. distros) beside steam. Linux gaming will be in general transformed to a vendor platform system (like android with Google play)... and also lock-in as Steam is NOT a open, community designed platform.
shaddim: Disagreed. It's pretty dumb for Valve to make Android style incompatible distro on a deep level (like using some other weird glibc, or reinvent a new display server instead of Wayland and etc.). They want their games to run equally on other distros too. So I expect SteamOS to benefit all other Linux distros in the context of gaming.
They want that their steam client to run everywhere, they do not want that Steam games run anywhere than their defined and wanted context (Steam platform). Important difference. If you look on the current steam client architecture, you see already this kind of thinking. They don't try to make the linux distro ecosystem more compatible they just try to make an isolated isle with own rules unhindered by distro rules, lib variations...etc. Exactly like android did, a platform on its own on top.