Yes, I believe after yesterday's announcement of the SteamOS, that we will not see a VALVe made SteamBox. VALVe will not risk their money on the venture of making a console-like PC and directly compete with PS4 and Xbone. And they are doing the right thing that will benefit both PC gamers and themselves.
SteamOS is a great idea. VALVe is great at making software, and I don't doubt SteamOS will have a good user interface, easy to use and friendly. The Linux based OS will probably be severely customized to avoid any traps other Linux-based distributions have, and it will be friendly enough to allow anyone interested in PC gaming to play at easy. I also expect easy to update both games and drivers on it.
Since the SteamOS is the most important thing, and it will allow you to stream games from your other PCs to it, I don't expect an actual SteamBox in the sense of a custom made microPC to compete with Sony and Microsoft. Yes, tomorrow we will see VALVe announcing partnerships with PC makers like AlienWare. They will license the SteamOS to anyone interested in putting it in their PCs.
What I expect for tomorrow is the reveal of a minimum spec PC to run the SteamOS and maybe some games. With that, VALVe expect to create a minimum spec PC for all games from now own to follow, making it easier to build a PC to attend the needs of PC gamers in a budget. We will likely see one of those minimum specs machines tomorrow, tough I have no idea if they will try to price it competitive with the PS4/Xbone.
VALVe played smart. There is few risks attached by just letting others making the PCs attempting to compete with consoles. Their OS will be popular with PC gamers making their own PCs, and any newcomer will be able to choose several configurations and prices that fit their needs. If they also include the basic PC functionality like web browsers, Skype and other stuff keeping it easy to use, I can see the PC gamer using SteamOS as their OS of choice.
I was wary that a SteamBox would have no reason to exist, as it would not find a place for itself. But by making the OS the real star, and letting others risk their money on this, I can see the SteamOS succeed big way, maybe even becoming a true competitor against Windows. Now I am really excited to see how it will work.
That said, I think is worth to be cautious too. I doubt the SteamOS will work perfectly at launch, and I would be wary of expecting too much out of it. Remember, there is not as many Linux games as Windows games, and if the SteamOS take its time to gaining market share, this will not change immediately, unless VALVe forces developers to develop to it if they want to be on Steam, which is a bad idea.
Also, there is the real chance of the first versions of SteamOS to not run as smoothly as expected, and to not attend all the needs of PC users at start. And again, those SteamBoxes could not convince people to drop consoles for them so easily, specially if they don't have the marketing push.
So, yes, SteamOS is a great idea, and it doesn't need an SteamBox to succeed. It could create a more unified and easier to use PC gaming scene, and I think it can succeed. But being a bit cautious is always good, and I will eagerly await for more details before passing final judgement. But for now, SteamOS is an interesting idea that I am glad to see happening.