(apologies for the cross-post)
The 5D c was "the original entry-level full-frame camera"
The 5DII was the upgrade of the original entry-level FF camera, to which they added movie mode almost as an afterthought & turned the cinematography world on its head.
The 5DIII is clearly *not* an entry-level FF camera; it's very much a professional camera that sits in the lineup with cameras like the 1DIV. It's got professional AF, professional construction & sealing (which is why it doesn't have built-in wi-fi & gps; the signals won't go through the all-metal body
). And a professional price...
But the 5DII is still selling like crazy several months after the 5DIII was released, because you can get a brand one now from reputable shops for a little over half the price of a mk3
(or a guaranteed refurb for ~$200 less than that, putting it comfortably under half the cost of its "replacement").
As noted here the other day, the 5DII just hit four years old. The supply contracts on some of the components are probably running out soon, which means that some of the parts needed to make it won't be available anymore (or at least not in the prices & volumes that they're used to). This would require redesigning circuits (& possibly firmware) to use newer components that replace the discontinued ones. I'm sure Fukushima didn't help the supply contract situation much either. The end result is that they have a near-obsolete camera (from a manufacturing perspective) that is still selling like crazy, which is not really a situation they've been in before in recent memory.
So they would be braindead not to replace the "just under $2K" full-frame position in their lineup with something in the segment that will continue capitalizing on the demand for an FF camera in this price range.
And it'll appeal to a wider audience than the 5DII it replaces. It's smaller, lighter, takes the SD cards that the cameras that a lot of people upgrading to it have, and has better low-light performance & better AF (which is why I'm getting one).
I think it's clear by now that the 6D is *not* in fact a reaction to the D600; if it was they surely would have put at least 7D-grade AF and the LCD viewfinder overlay screen in there.
The 6D is the replacement for the 2012 5DII.