A 5Diii with 7D build, AF and speed would seem to be many people's dream come true....If a 5Diii was virtually identical to a 7Dii except it had more megapixels on a larger sensor
Yes, it would be a dream come true - except that it's a dream that will go 'pop' and disappear when everyone wakes up in the morning (probably some distant morning) and sees the specs of the real 5DIII. Hey, I'd like a 5DIII with better-than-7D build, better-than-7D AF, and better-than-7D speed, still full frame, and I'd like it to cost about the same as the current 5DII. Oh, wait, except for that last little point, that's the 1D X. See the problem with comparing dreams?
As Flake stated, people are interested in the 5DIII in part because it's been a while since the 5DII came out. The assumption that a 5DII replacement will have '7D build, AF and speed' is unwarranted, because Canon will need to differentiate the 5DIII from the 1D X on features to justify the price difference of ~$4K (or they'll minimize the price gap and charge $4K for the 5DIII but I really don't see that happeneing). So, people won't get their 'dream camera'.
Same for the 7DII - no one's talking much about that one, but it will probably not improve too much on the 7D. My guess would be more MP (21-24), maybe the articulating display, same basic AF, Digic V (likely x2), new metering sensor (which will contribute to AF, so that will be the AF improvement), and that's likely it.
Instead, let's compare real cameras - 5DII to 7D. I think Fleetie summed up the main advantages and disadvantages. For me, the 1.3-stop better ISO noise performance is the key - on the 7D, ISO 1600 is barely tolerable, on the 5DII, ISO 3200 is decent.
The difference between the two might be better expressed as uses rather than technical. 5D MkII better for portraits, landscapes, and architecture, perhaps some low light work.
7D better for sports, and action, wildlife, macro
Flake is correct - it comes down to use cases, more than tech (although I disagree on the macro part, where the 5DII is a better choice than the 7D - 1:1 is the same size regardless of sensor, and I'd rather have a 1:1 image captures a 2.6x larger area).
I had the 7D before getting the 5DII. Since getting the 5DII, 90% of my shots have been taken with the FF body. The 7D has been used almost exclusively for birds/wildlife shooting. If all I did was birds/wildlife, I would have neither the 7D nor 5DII, but a 1D IV - and I'd keep it over the 1D X for that use. But like briansquibb, I see the 1D X as replacing my 5DII, and I'll be keeping my 7D.