And, with all due respect, the implication that owner's bias trumps the experience they have through ownership is fallacious. If I own something that isn't actually good, I quickly sell it and replace it with something that is. It's that simple.
Without putting too fine a point on it, in my very humble opinion you are mistaken because there is no alternative to the 6d with Canon and in this price range except for the outdated 5d2. So what would someone who's unhappy with the 6d replace it with? Sell all glass and buy Nikon? Spend €1000 + 6d loss more and get a 5d3? See - it isn't that simple. Plus many people are simply attached to things once they bought it, I know I am.
I have an issue with this, because, its not as if there is no alternative, just not in this price range. it's like that with all things, you get this for that, and if you want more you have to step it up to the next package. To use my own business as an analogy - my mid level package offers ceremony and full reception coverage, but if you want ceremony prep, you move to the next tier, you want a second shooter, you move to the next tier. Now I am not as stringent as canon, and, for me its not like i have to retool my whole factory just to offer a compromise in my packages, but either way - the analogy holds true. You can't act like there are no other options - because there are! Even in the 6d price range or less, you want FF but don't have the dough you can find product...i see the 1dmk2 and the 1ds at about $650 - 1dsmk2 for around $1500, and there are 1dsmk3 floating around the $2600 range.
So, you only option is far from selling you gear and moving to nikon, and real world ---the d600's only real
AF advantage is in servo mode because of the extra points, but the spread of those points is about the same - so if your trying to shoot sports on a budget or wildlife and need tracking, then you want a 7d or a mk3 or a d600. But, the d600 offers no advantage for the "low light fast prime lens far corner composition and I don't want to focus and recompose or crop in post issue."
Back to my original issue with your comments though, it sounds like you just want a mk3 for the price of a 6d, and it also sounds like you think the d600 is more on par with the mk3 too, which isn't true. Many of the 6'd shortcoming are in the d600 too:
Max shutter speed - 1/4000
max sync speed - 1/200
memory cards - SD
And - native ISO on the d600 is 6400 ---you have to use the expansion to boost it to 25,600.
So, the d600, while yes it is a fine camera and I wouldn't hesitate to recommend it to any shooters who currently shoot with nikon gear, but ---I wouldn't recommend switching systems based on AF alone.
Again, kind of sounds like you want to have your cake and eat it too. With all things in photography you have to make compromises unless you have an unlimited budget. Like when i made my 70-200 decision, I couldn't afford the IS version at the time ---compromise, the f4IS and the f2.8 non IS were right in the same ballpark - so I made the compromise of losing IS in favor of the 2.8 aperture. When I bought my 6d, I was looking for a backup body that could hold its own in most situations with my mk3. My budget just couldn't afford a second mk3, so I compromised and went with the 6d. This list can go on and on...we can't expect canon or nikon to tailor make our products to each of our personal needs and desires. So we balance the decision based on wants vs needs vs available budget.