The 5D release cycle looks to be between 3-4 years, so the idea Canon will be releasing a 5DmkiV next year seems premature. Considering the 5DmkIII, along with the 6D, are selling quite well (according to Amazon's best selling DSLR list), it would seem more likely Canon will continue milking the profits from this camera for 1-2 more years at least.
The D800 and D610 would be Nikon's best selling full frame cameras, and while they don't seem be be selling as well as the Canon FF models, they are doing quite well themselves. So I would seem off base to call them a flop. I know a lot of D800 owners personally, many who shot with D3/D3s/D4 have it as an extra body for times when more resolution is desirable. I've also known many who have owned one and sold it, so retention after the original sale may be a problem with the camera. It is a unique camera, and either it meets your needs or it doesn't. If it meets your needs, then there is nothing else that will. The Df is an interesting model, though. It already seems to list high on the best seller list. It will be interesting to see if it stays there. It's got a fantastic sensor, and many unique features.
But Nikon's line up seems a bit fractured, I think they have missed the boat on making a singularly popular camera, like Canon has with the 5DmkIII and even with the 6D. What the demand for a higher MP camera from Canon is really would be a hard question to answer. Maybe Canon hasn't found the market would be strong enough to support such a camera from them. Or maybe they are still looking to get some updates to more of their lenses in place to support a super high MP camera. It has been reported that next year is the year of new lenses from Canon. That would make sense as there isn't much of a need for a new camera in the lineup other than a high end, high MP model. And to release such a camera before there were sufficient lenses to really take advantage of the added resolution would probably hurt sales ultimately. There is probably need for improved wide angle zoom lens, something to compete with Nikon's 14-24. If Canon does release a high MP camera, the current WA lenses are not likely up to par to do such a camera justice. The 24-70 & 70-200 mkII models are certainly good enough, but the 16-35 or any other wide is in desperate need of improvement to bring them up to the same level as those lenses. The line of newer prime IS lenses could be expanded to include a 50mm, and maybe a few more, like the 85/100/135, and maybe the 20mm too, to round out the lineup.