Lets look at history. When the 5D came out, it bettered the 1DS Mark I. It bettered the 1DS Mark II except for resolution which is just a tad better with the 1DS Mark II. I had the 1DS Mark II for nearly a year, then replaced it with 3 5D's for the lower noise of the 5D's and just a small step down in resolution.
So if history repeats it's self, then the replacement to the 5D will provide BETTER IQ then the 1DS Mark III. This has happened before and I think it will happen again. And based on that, I guess the 5D replacement will be 16-20 MP, and provide lower noise then the 1DS Mark III, higher ISO's too.
Will the replacement 5D kill 1DS Mark III sales? YES, justs like the 5D killed 1DS Mark II sales, yes, of course!
Canon is not stupid; they know this, and they are going in resolved to this. And they prepared for it by introducing the 1DS Mark III BEFORE the 5D replacement in order to get as many Mark III sales as possible before the the 5D is replaced.
The primary things one gets with the 1D series over the non-1D series is features, build, sealing, FPS, dual flash cards, better ergos, and much less often to do with offering better IQ. This is something that is very hard to accept by owners of 1D series bodies. I had two 1D Mark II's and one 1DS Mark II and I can easily write this. The 5D is that freaking good. Still, I'm looking forward to the 5D replacement and will probably get two of those over a 1DS Mark III.