Lol, you talk about cross-buy as if Steam doesn't even do that already. The whole point of SteamOS is that games you buy on PC or Mac versions of Steam are already available (assuming hardware support). Even the Playstation version of Portal 2 was effectively cross-buy. Nearly 1000 titles are currently available for SteamOS before its even reached a full commercial release and includes some big PC franchises like Civ Beyond Earth, Total War, Metro, Football Manager and all Valve games.
You say SteamOS will likely never be popular but people were saying that about Steam as well. I see it experiencing slow and gradual growth, which is actually all it needs. You keep your library regardless of hardware. It's the same idea as Android Play Store/iOS app store and in the long-term it works.