In many ways, the 5DII 'butchered' the 1DsIII sales, and many 1Ds customers got a 5DII instead for the video and other reasons. I agree that Canon will want to avoid that in the future.
The thing is, as the saying goes, you can't have your cake and eat it, too. Canon does have to differentiate the 1D X from the 5DIII, and there are a couple of ways to do that. One is time - the 5DIII comes out a year, or more (!) after the 1D X. The other is features, and especially if the 5DIII is going to have more MP, count on it being substantially crippled in other ways - most likely frame rate and the AF system.
The 5D series is never going to have more than 5fps - it may even stay as low as 3 or 4.
Further, it could have fewer AF sensor spots and it would make no difference to *me*. If I mount it on a tripod, I don't even use the AF sensor at all and probably never will again - with live view I can precisely select where I what I want to focus.
I also wonder if the build quality will stay the same - i.e. no weather sealing on the 5D3. That won't upset the folks using the camera to shoot video, nor those in the studio, just the hoards on the internet forums that won't buy it because it isn't 100% weather sealed (well, they wouldn't have bought it anyway, but if they want to pretend, why should we stop them?)
But if the price was $3000, would people expect weather sealing?
Would they expect dual card slots?
Will it sell without them?
How does the up coming Sony A9x and whatever price it gets launched at change things?
If Canon splits the 5D and bring out a FF DSLR that undercuts the A9x on price, how do they keep it, the 5D successor, the 1DX and 7D successor all different?