Interesting points, Dilbert but it still feels to me like a lot of work and cost to Canon to fill an awkward gap in the line up that perhaps doesn't really need filling... or at least may disappear in a year or so when the 5D3 price starts to drop or the 7D2 comes up from the rear.Here's the thing...the cheaper the camera gets, the less the consumer will have researched little details, and the more they'll want the newest thing. They are also the lifeblood of companies, because hooking someone in at the entry level is how you get them upgrading and sticking with the company.
Although the 5D2 is getting on a bit the IQ does still fits between the 7D and 5D3. Just my thoughts anyway and you're of course entitled to your own. I do however think compatibility with the latest accessories is a valid reason to end the line... or at least push out a firmware update.
So, if Nikon releases a D600 with something pretty close to the reported specs at say $2000ish, then the 5dII market is gonna dry up fast. I can't imagine the person who would want a Ti level auto-focus over even the 7D autofocus, or a 4yr old sensor vs a brand new sensor. They'd want it even less if the cost is <$2000. And an entry-level full-frame is where you hook your high ticket buyers. You go full-frame, you start buying L glass...
The D800/5dIII thing won't affect Canon, but not having an answer to a D600 would.