Well, consider this.
If the PS4/Xbox One were to underperform, what incentive would there be for Sony and Microsoft to stay in the market?
The PS3 cost Sony in the region of $5 billion dollars, and Sony is gambling nearly everything on the PS4. They simply cannot afford to let the PS4 fail. If it does, I can't see there being a PS5.
And then we have Microsoft. They saw the Xbox brand as their way to dominate the living room, but it's quickly becoming obvious that the television doesn't matter as much as it used to. Google and Apple have come in and won the living room without even trying. I can easily see Microsoft leaving the gaming market if the Xbox One underperforms.
And, of course, there's the publishers, too. They follow the money. If that's in tablet/smartphone gaming, that's where they'll go. Just look at Japan.
I think you forget that America != the world.
PCs are very popular in Europe, Asia, South America, and a lot of other places. I remember Valve saying that there's more Dota 2 players in the Ukraine than there are in the US, despite the US having nearly 10x the population, which is nuts.
Steam currently has 65 million active users, and that figure's growing every day. I'm betting that there's a lot of people out there who want to use Steam but are intimidated by the PC, and Steam Machines will fix that. I think saying that 'Steam Machines will have no significant impact on the market' is very shortsighted.