Future Xbox360's will not be radically different....smaller box due to smaller components, larger hard drive, different colors...maybe Blu-ray. But there will be no performance or functional changes. The oldest or cheapest version of the Xbox360 will have to be able to run all the same games as the newest version of the 360. That will dictate what does and does not get added to future revisions. It's as simple as that.
Now, if we start thinking about the next Xbox, that's a different story...if there will be one, that is.
Now, if we start thinking about the next Xbox, that's a different story...if there will be one, that is.