And, for that matter, why would the 5DIII need to be replaced?
To get 3 stops more low ISO DR and get rid of low ISO banding?
To get a nice 4k?
To get crisp 1080p without RAW?
To get more MP?And, for that matter, why would the 5DIII need to be replaced?
Dual pixel, new RGB+IR metering.
These might be nice, but as someone who actually owns a 5DIII, I don't find any of these to be compelling reasons to upgrade. Judging by some of the comments and recent polls on this site, I'm not sure many other 5DIII owners are feeling a need to upgrade either. Waiting another year or two for more significant technological improvements might make more sense.
What more significant improvements? If 4k isn't anything, nor crisp 1080p, nor firmware with aids that actually let you focus video and see blown highlights, nor better metering, nor dual pixel AF, nor 3 stops DR at low ISO, nor 1+ stops DR at high ISO, nor 46-46MP, etc. what are you expecting?
First, I'm not sure your wish list is realistic.
Second, I'm not a video person, so neither 4K nor 1080 p has any interest for me personally. (Although I recognize that it might interest video people) but not convinced it is such a necessity that some people think, given that now and for the foreseeable future most video is going to live on the internet.
I can see blown highlights without the camera telling me.
I'm regularly amazed at how good the existing Canon metering is already. Even when I bracket to compensate, the camera's chosen exposure is almost always spot on.
I don't use live view, so dual pixel has never been a big interest.
You should know by now that low ISO dynamic range isn't a big deal to me. (I'll take it if given to me, but I'm not interested in paying for it.)
I'm always interested in high ISO improvements, but pretty pleased with the 5DIII's performance as it is.
And, for me, 46 mp is not a plus, it's a negative.
Now, I know that's just me. But, reading a lot of comments on this site from a lot of 5DIII owners, I think I may be more typical than you suspect.
So, what would interest me?
Lightfield technology, offering post-exposure focusing;
Throw in user friendly wi-fi and touchscreen, higher frame rate, an in-camera photo editing program (especially one that can be linked to an iPad) so you can edit images, convert them to jpeg and post to the internet all without having to first transfer the files to a computer, better weather sealing, and a few other improvements we haven't thought of yet.
Aside from the Lightfield technology, which may be a few more years away from making autofocus systems obsolete, the rest of this stuff should be either available now or available in the near future.
In the meantime, my main point is simply that I'm content with the current 5DIII and don't see much that would make me upgrade.