That said, the volume of 1DX sales is by far the smallest of the three. It would be easier to make enough 1DX sensors for the foreseeable future and keep them in storage. My bet would be the 6D gets replaced, the 5D3 gets replaced, and then the 1DX gets replaced last..... and that's a long wait!
Agreed. I would expect the 6D to be updated on a schedule that's slower than the rebels, but faster than the higher-end pro bodies. My money was on approximately a two-year cycle, which would make the 6D mark II about a month and a half past due. I would not be at all surprised to see it updated "real soon now".
With that said, I would expect the 5D to be on a three-year cycle, which means I'd expect it in a couple of months. So if I'm wrong about the two-year number, it could come out first. Or they could shock everybody by releasing both of them in March.
In my mind, 6D was some sort of bastard hybrid between FF and crop. It seems to me they felt there was gap between lowest FF (5D3) and best crop (7D), and they wanted to add something in that gap. So they took some stuff from FF level and slapped in things from crop level, and voila, you get 6D.
It had lot of good features too, but the AF alone was a show-stopper for me. My shooting I need higher end AF and 6D was not good enough. For many other purposes, it's same as 5D3.
So my guess:
5D4 announcement soon, maybe in <2 months. Followed by 1DX2 announcement 1 month after that.
5D4 shipping end of summer ($4k), 1DX2 ($8k) shipping end of year.
And early next year the strip-down version of 5D4 -> 6D2 for $2.5k.