Totally different markets? I doubt that. They'd have to nerf the 5dmrkiii pretty bad for it not to cannibalize any 1DX sales. I agree with neuroanatomist when he says that "time" is a good way to differentiate the two. Plus the mrkii is still selling well. So for all of the reasons mentioned, the probability of getting your hands on a 5Diii within a year is very low.
If Canon don't release any other FF camera in the near future the 1DX will obviously benefit as some may buy it even if its features don't totally suit them but I don't see that as "cannibalizing" 1DX sales if they release something more suitable for those users.
The markets just seem far more different than the 1Ds mk3/ and 5D mk2, there you had two bodies sold on MP first with the higher end one also giving some of the 1D functionality(decent FPS, high end AF, high end double grip build). Not everyone may have been happy with the 5D's AF and build but equally the 1Ds mk3's AF and build/size plus the prenuim they came at was probabley overkill.
But for the sake of discussion, say they did want to differentiate it with fps. Would people be happy with 36MP at 2fps(slower than its predecessor)?
The 1DX processor seems to hint that 3 FPS for 36 megapixels which would I'd guess keep most users happy and make sure it stayed well away from the 1DX.
If your getting below 3 FPS then I'd say that would depend on the cameras market, if its the only FF body besides the 1DX thats perhaps too low but if theres also a lower megapixel 5D sized body with higher FPS then the ultra high megapixel body would I'd guess be mainly selling to the landscape/studio market were FPS isnt that important.