I think Valve did a smart move (but a tad too late):
The gaming market is simply changing, and as MS and Sony try to adapt to that Valve does too. With games on mobile device slowly outgrowing their casual roots, and Android devices like the Quya pushing into the living room, making a console isn't exclusive to big manufactureres anymore. This also pushes the current tendency to digital distribution of other medias (Video, Audio) even further, as installing and keeping an up to date client is as easy as getting a new game.
And this is also the market where rather big grow rates are still possible, the console market itself is already saturated (with mobile devices chewing off a small portion of it). But there are still a lot of people that not necessarly want a console in their home, but are interested in media streaming, and may buy a game occasionally. Or the avid PC gamer that usually prefers his mouse and keyboard combo, but may be interested to play a new plattformer rather in his/her living room. With media pc aren't something really exotic right now, there may be some of them that replace it with a Steam OS system, or by one instead of making a custom media pc.
MS and Sony are pushing in that direction too, just to broaden der posibilities of income.
But compared to MS and Sony, Valve has no need to rush. They can slowly trickly into the market, as they don't have the burden to release a new console generation. Their basic "system" is already up and running, the hardware is just a byproduct, which hasn't even a big impact on their whole system. Consoles are having the problem that their hardware is already outdatet shortly (if not even before) their release, but with Steam it is mostly up to the console owner wether he wants to keep is older system or buy a new system to get neater gfx. Which is another thing in favour of Valve: No need to decide wether to keep the old system to play some old titles, or just sell it- they will continue to work with the new system, which gets interesting if you are into online multiplayer gaming (not everyone of your friends wants to keep his old console too).
MS and Sony may get into that too, as they are relying on more standard pc components, but they still got their Plattform/their market share to defend, which means that they need to push exclusive titles, which will cost them. Valve is already a big player on the pc gaming sector, so they can just observe how the market is evolving and react accordingly.
The lack of games maybe a drawback at first. But with MS and Sony building standard components into their consoles this will fade away in the future (except exclusive titles of course). Big publishers tendency to spread their titles over several plattforms isn't getting smaller, and with a lot of middleware already doing the gruntwork here, I guess it isn't a tremendeous effort to bring a PS4 title onto SteamOS.
Indie titles maybe already cheap on the xbox, but "industry" (whats the opposite of indie ?) titles aren't, but looking and this whole "sale" thing steam is doing on a regular basis, I guess it won't be long as this isn't a big factor either.
The controller isn't a big "dough" either. Valve at least tries to adapt to the problem that is at hand if you try to play games that are developed with mouse and keyboard in mind without it. If it is no good, who cares. There are literally thousands of alternatives readily available, no need to buy extra pricy accessories (like you need to with MS and Sony).
As much as I don't like steam, I hope (and think) they will chew off a big part of that market, forcing MS and Sony to rethink their strategies.
No hard data so far to support this assumption 
