Full size uncropped image. Autofocused with the 6D center point on the yellow bubble level, then recomposed. Apparently nobody else can do this, but my camera can while I'm bending over, and handholding it in a very dark room, at 1/13 second, f/1.8, 24mm, about 4 inches from the lens front element, with no image stabilization. It looked darker than this to my naked eye, and I don't think I used exposure compensation (the metadata doesn't show it if I did). Shot only as a jpeg, with some NR applied in LR. ISO 25,600.
What's all the fuss about focusing and recomposing? If little old me can do this, hand-held with no image stabilization, why can't others do it at 1/200 second in good light, or especially with strobes or flashguns? Not saying all lenses and focal lengths are the same...but gee whiz. It's not unheard of, because I did it.
The 'fuss' is that focus/recompose causes backfocus. It's simple geometry– the focal plane is flat (field curvature notwithstanding), and you're rotating the camera after locking focus, which moves the focal plane to a position behind the subject after recomposition. With a narrower aperture, the deeper DoF is often sufficient to mask the effect of the backfocus, but with a wide aperture, you'll see the backfocusing.
Frankly, your image of the ballhead is so dark and noisy that it's difficult to say anything is in sharp focus. But one thing that seems at least close to in-focus is the index mark on the left side of the clamp, and that's well behind the bubble level. That index mark is certainly more in focus than the lettering on the front of the clamp, despite that lettering being much closer to the bubble level. So if you did indeed successfully focus on the bubble level, then you've demonstrated (albeit poorly) the problem with focus/recompose.
It's not that 'nobody else can do this', but can ≠ should, and many of us know that focus/recompose causes problems with fast lenses shot wide open. If you're using a slower lens, stopping down your fast lens, or aren't a stickler for critical focus on your intended subject, focus/recompose can work.
You're conveniently ignoring the angle the shot was taken at. The plane of focus is on a double diagonal, relative to the plane the quick release plate exists in. Look at the area around the bubble level. What is immediately in front of it, is not in focus. What is immediately behind it, is not in focus. Sure there's noise, but I was making a point about the low light autofocus ability of the center point, to myself (I shot this in January). The 5D3 and 1DX, would not have autofocused at all in this light.
And of course the bubble level is not razor sharp. But consider the conditions. 1/13 of a second, at a distance of 4 inches, no image stabilization. The point is, the bubble level is in the plane of focus. You can try to deny it, but I'm sorry, it is.
Surely you're not saying the plane of focus at f/1.8 and 24mm, at a distance of only 4 inches, is more deep than the plane of focus of a 50mm f/1.2 lens, at a distance of 6 feet or greater (or whatever the average portrait distance might be the choice for such work with that lens)?
I'm not saying what you say is not correct, regarding the problems inherent with focus recompose technique. Also not saying it would automatically work as well with the 50L or 85L, with the 6D, because I have no experience with those yet, on it. However, other people who have posted recently, have related their efforts with the 50L on a 6D, and at least one has said they use "focus-recompose". If you have a problem with the technique, take it up with them. In any case, you have almost no experience with the 6D, thus your contributions to this thread are less valid, than those with more experience with the camera.