I'm sure Steambox will be as successful as the Xbox. Just as soon as they get all of the major studios to support their platform, make sure that the hardware is comparatively cheap to the Xbox and the PS3 (i.e. by selling it at a loss), and by aggressively standardizing and branding the device hardware.
They'd also need to get some exclusive AAA titles (like how Xbox has all of Fable and Halo for instance) in order to enter the market, and let's not forget that getting exclusivity on an AAA game for an entirely new console means writing the game yourself, or buying the company that makes it (Fable and Halo are both MS).
So yeah, if Steam was willing to have games that runs on Steambox but not Steam (since that would dilute the exclusivity of the titles on their console), was willing to buy other games companies and put billions of dollars into entering this market as a level-player in the console business, then yes, maybe Steambox and Xbox 360 might be a viable comparison.
But until then, not so much.
I agree the chances this will unseat the major consoles is pretty slim, especially considering the billions of dollars of investment the major consoles spent to get where they are..
But when you structure your business model such that you don't need to sell millions of consoles to make a return, it doesn't really matter. Like if I make $200 per console and my initial R&D was $250k, all I'd have to sell is 1250 consoles in total to make my investment back. I'll be more conservative at say $1 million initial R&D investment, which can easily pay for like 7-8 full time engineers working a whole year on the project, and you'd still make your money back with selling 5000 consoles.
It's hard to say how popular this thing will be without knowing what Valve is planning though. One thing I can say is it's pretty silly to underestimate Valve's capabilities to make stuff people will want to buy, and unlike their competitors they do it without much of any public capital. The company is practically legendary in the games industry, and they show they can start entire new market without even trying (Steam).