I'd postulate that it's more of a marketing thing rather than a hardware thing.
Consoles are designed to be simple, convenient and cheap. Perfect for the "casual" gamer or someone who just wants to get to the games quickly and easily. The mass market. To people like us who have in depth knowledge about the industry and the hardware, what you've described seems like an obvious choice and obviously a good thing, but I think the average consumer probably wouldn't understand it unless it was explained very carefully to them.
I think the market for this kind of thing is actually pretty small. Most consoles users like their consoles for being quick, easy, and cheap. Most PC users want the most customization possible. This kind of system would really only appeal to a very specific demographic that wants a little bit of customization, but not as much as a PC.
The other thing is that it's not a good business strategy for consoles to let users upgrade rather than buy a whole new console. Unless Microsoft is locking out all competition except their own first party graphics cards, they won't profit from users upgrading, and they won't profit from new console sales that would have replaced upgrading. As devious as it seems, I don't think it's a smart choice as a system designer to lengthen the lifespan of your system. That is a great feature on paper, and might convince more people to buy a system at first, but might also limit your revenue later in the system's life.
Another issue was brought up by Jim in a Jimquisition episode called something like "The irony of PC gaming" in that graphics are the most commonly brought up advantage of PC gaming, but the real advantages are the library, the customization, the modding, the indie games, etc. Practically everything but the graphics are real, solid, objective features the PC lauds over consoles. The graphics should simply be seen as the icing on the cake, not the main advantage.
Anyway, that's why I think it might not work. HOWEVER, if Microsoft can make the Kinect of all things sell, fucking ANYTHING is possible, and they might just be able to pull it off.