There are some distinct differences between Value's situation and the examples you provided.
1. All three of those successful consoles from outsider were price competitive within the existing market. If you look at the literal pile of failure consoles throughout history they were all substantially more expensive, and so far what we know about the Steam machine says it will be substantially more expensive.
2. All three of those successful consoles from outsiders were presented as a singular hardware model by a single manufacturer there was no significant hardware differences from one model NES to the next or one Xbox to the next, Valve is planning on developing more of a spec and opening hardware development and marketing up to multiple hardware manufactures... a strategy used by several consoles in the past (most notably the 3DO) to great failure.
3. All three of those successful consoles from outsiders launched with a strong list of exclusive titles that you couldn't get on any other platform. Valve finds themselves in the situation where anything developed for the Steam Machine will also be available on PC, without the machine, this inherently makes their hardware less valuable as there's literally NOTHING it can play that couldn't also be played on a PC or elsewhere.
In general the three factors that historically have contributed to a successful console (by a new entrant into the market or otherwise) has been price competitiveness, and desirable exclusive games, as well as a desirable feature set (such as the PS2's DVD player or the Wii's waggle controls, or the 360/PS3's ability to play games in HD). So far the Steam Machine seems to be missing the mark on most points.
Then again, the iPod looked like a turd on paper when it was released and that thing sold like gang-busters so who knows, stranger things have happened.