I really don't see how you could make a full frame "starter" camera. Remove AF adjustments? Instead of releasing a new camera, why not just sell the 5D MK II for cheap?
Have to agree, the tooling are already in place, perhaps just update the AF?
Thing is, there is a point of diminishing returns there. Canon set up the 5dII to retail at $2700 originally, and then dropped it to $2499 for a while. Now its basically going for $2000 every place, what price would they have to drop it to to compete with a D600? If that price is $1750 or $1500, you have to imagine there is a point where the margins get so thin that they aren't making money. Plus, there's the theory of "newer is better" in the camera world. A 5-yr old design competing with Nikon's newest is not the way to snag new customers.
Plus, if the change the body to more plastic, it can make it cheaper. Likewise, making use of the 5dIII or 1dX sensor condenses that part of production. Right now, they have to actually produce 5dIIs as none of their current line uses any of its features. A frankenstein of current pieces is probably more cost effective.
Basically, they can't sell the 5dII at $2000 for the next 3 years...but a 6D they probably could, even if its only the minor upgrades mentioned