my bet is the 5d3 will get the standard cycle of 3-4 years and the reason this is being brought up is to get people talking - because while there is a ver vocal minority of haters for the 5d3, there are tons of happy users who stay quiet! A high mp camera with better DR is geared towards a different user base than the 5d3, which yeah, appeals to most wedding shooters like myself.
Again, in my local photographer community, no Nikon wedding shooter recommends the d800 to those looking to upgrade - most say d700, d3, d3s. I know many will flame me for saying it, the good old - memory is cheap argument. But, those wedding shooters that did snag a d800 say it stays in the bag on wedding day because the files are just too damn big and adds too much time to the work flow. (it's not just buy more CF cards and new HD's, most will have to upgrade their computers to handle the larger files).
With that said, a canon high mp body may be more versatile because it would most likely offer RAW, mRAW, and sRAW - where d800 only offers crop mode (I'd rather have a lesser mp'ed image at the full scale of the FF sensor than a smaller file but cropped). So on that end, a high mp body may sell to wedding togs. But, I doubt wedding photographers woud leap at such a body (especially if its in a 1d series body), maybe a few really high end wedding togs (many of them may buy it for the studio and leave it there on wedding day). Even if it has better DR, how much of that would be lost when shooting at mRAW or sRAW? Thats why I think the 5d3 hits the sweetspot of performance across the spectrum.
Last thing to say, a split line for for the 1D and 5d does make sense - wedding and sports shooters would still have their tools, while studio and landscape shooters would be abe to use the 1dxs or the 5d3s.