Sometimes XDCAM just hasn't the colour depth.
Sometimes a Digibeta or HDCAM is just too conspicuous.
Sometimes a Red runs too hot.
Sometimes a GoPro just isn't enough.
Every camera has concessions. And the major concessions the 6D has, are nowhere near solved by having a 5D3.
It used to be the 5D2 bores. Glad they've been more upwardly mobile this time. If you are a 5D3 user and you want to pith all over the 6D's parade then have fun. The guys shooting on C300's, Sony F's or Arri's haven't even noticed you are in the room.
It is all relative. If somebody is capable of getting good video out of a 5D3, then they will be capable of getting good video out of a 6D.
Both the 6D and 5D3 are good video cameras, but the superiority complex 5D3 users really are having a big old laugh to themselves when it comes to moire etc.
I strongly disagree. The 5D Mark II remains a very nice camera, but its low light is poor, the codec has issues, and for wide shots in particular the aliasing can be terrible. Everyone knew this even while they were using it. For certain jobs it's just as good as a Mark III, and it's a viable camera for most purposes, but I feel confident walking in with a Mark III that I can deliver everything a client expects, except resolution on par with high end systems. With the Mark II, I'm worried about wide shots, low light, fabric, and hair. Projected large, there's a difference in wide shots in particular. Granted I found the upgrade from a t2i I owned (and 5Ds and 7Ds when available) to a 5D III to offer depreciating returns, but all the major problems (noise past 800 ISO, unacceptable amounts of skew, unacceptable aliasing, etc.) are tempered. Saving $1000 and getting a 6D for video and suffering many of the same problems seems foolish. That said, upgrading from a 5D Mark II might be very unnecessary depending on your needs. It sounds like your needs don't require the strengths a Mark III provides over a 6D or 5D Mark II. But there are people for whom the difference is getting a shot and not getting it. Technically all three cameras can produce visible aliasing, but look at the video posted above. Only one camera provides consistently acceptable results on fine patterns.
Furthermore, none of the camera systems you've listed (digibeta and go pro?) are comparable in any way to dSLRs and wouldn't be used for the same productions. Apples and oranges. Whereas the 6D and 5D III are like... apples and apples that are much better for certain types of video.
The F3, Epic, and Alexa are likewise suited for a different kind of production. It's not like they're even better or worse (in terms of IQ, they're better, though the Epic has mediocre low light), they're simply designed to be operated by more people on a bigger set, provide a more flexible image, and interface better with a traditional crew. They're designed mostly for TV production and theatrical features. dSLRs are more crash cams and web video. There are jobs for which a dSLR is much better than an Alexa, and even if you're lucky enough to be hired for narrative production, I doubt you'll encounter many shoots without a dSLR tucked away somewhere grabbing the odd insert.
I'm glad you're off in Alexa land ignoring us dSLR shooters who are off your map, but there are shooters who won't find a 6D useable and who will find a 5D Mark III great. There are also users for whom the difference will be virtually unnoticeable. For talking heads (and assuming no issues with moire in hair or fabric) both are just about as good. For wide shots, fast motion (skew and the ALL-I codec matter), and low light the difference is worth more than $1000.
To some people. Just as the $60,000 step up into Arri land is likely worth it to you! (And I'm in agreement, the Alexa is just the best thing going for narrative production, not that I can get hired to shoot on one.)