Ok, I'll respond the way I do in most of these next gen discussions (copy/paste time!):
As I said before many times, Nintendo was in an interesting position: with their addition of HD, improved online service, better third party support and new controller, Nintendo can easily justify a new console. And that's the key word in this matter: justification.
What can Sony and Microsoft do that both A: Can't be done on their current console and B: Can justify the price of a new console? Besides improving the graphics, what can they honestly do? And keep in mind that the next consoles will likely be more expensive than the last, or at least just as expensive. So what could Sony do to justify players paying at minimum (everyone together now!) US$599?.
Here's the main point of my argument: A graphics update is nice, but every console is doing that. And here's the thing: by this point in time, there's not a big enough jump in graphics for it to justify the price of a new console. This isn't 2D to 3D. This isn't SD to HD. This isn't even 8-bit to 16-bit. This is where things get interesting for Sony and Microsoft: at this point, there isn't a big enough significant difference in graphics for it to be a justification for a new console on its own. So what can they do?
Nintendo, while probably not guaranteed to be the best seller next gen is certainly in a very comfy position. After the Wii, pretty much anything positive they do can be an attempt to woo back the hardcore gamer, and they can focus actual marketing of the WiiU to the casual crowd they build with the Wii into a more "hardcore" gaming system. Not everyone in either crowd will buy the WiiU, but a fair amount of both is all they really need. Are they guaranteed success? No, they aren't. But in terms of the next console, they at least have a good reason to make one.
Sony and Microsoft only have one audience: the hardcore gamer. And as stated before, there's not enough of a graphics improvement to be the selling point this time. If they go with graphics alone, it eliminates perhaps the biggest advantage consoles have over PCs: not having to constantly update for minor issues and constantly having to keep up with technology and requirements. A new console with just a graphics boost really at this point can summed up cynically as "you need to meet the new system requirements". Microsoft tried to get a new audience with Kinect, but their bread-and-butter, the hardcore gamer despises Kinect. Same with Move. Now there are rumors (and just that rumors, I can't prove it) Sony putting in a Kinect-Like controller, while it COULD count as justification (I don't think so personally, why not just make a Kinect knock-off for the PS3? But that's just me.), they also run the risk of losing a bit of the "Hardcore gamer" base they've essentially spent this entire generation living off of.
Fanboyism can only carry a system so far, just look at the Gamecube. So what exactly can Sony and Microsoft do? They either lose the main advantage consoles have over PCs, or they try something different? However, with the exception of graphics, we have learned that one of the most hated things among Hardcore gamers is "different" (in terms of hardware, anyway), so there might be the risk that they may, like Nintendo before them, may alienate the hardcore audience. Now am I saying they're doomed? Of course not! I don't know what they're planning, they might have found the next Wii Sports (I'm speaking in terms of system sellers) for all I know! I'm just saying that the odds of them releasing a console at this point that can A: Be more than just a graphics update B: Not risk alienation of their hardcore audience and C: Have features that simply can't just be added to their current console seems pretty damn steep.
