The point made about the 50D not having video was the big reason I think it was not a major hit on the market, and because the AF was not upgraded. To say the 5DmkIII will have similar market irrelevancy is a hard comparison to make, the only thing I can see holding the 5DmkIII back from record breaking sales figures would be the price. That is the one thing Canon can address, if they feel the need to. I'm sure the price will come down some eventually, the question is how much? The answer will be how many does Canon want to sell?
I think the other aspect is what does Canon have up their sleeve? A ~$2k full frame camera may be on the horizon, and that camera would probably be a huge seller. I think Canon hurt some of their potential 1Dx sales by making the 5DmkIII so good (and by getting it to market so quickly), so they may decide to keep the price up on the 5DmkIII to offset some of the lost sales on the 1Dx. The new, lower cost full-frame will probably be the one they look to become the mass seller, pricing it more competitively to get record breaking sales figures. Does that mean the 5DmkIII will be irrelevant? I doubt it because it has all the top features, the less expensive model is sure to lack some of those, making it cheaper to produce and less desirable to those who require all the features. I would suspect the cheaper camera to not have the best weather sealing, a single card, maybe the AF won't be quite as good...sounds a lot like the 5DmkII, don't you think? Maybe it will have lower MP than the 5DmkIII, but the same AF. Whatever it is, I think you can count on the 5DmkIII still having better specs and still having a good place in the lineup, Canon has had a long time to bring that camera to market, and I'm sure they thought about what they would do to make sure it had adequate shelf live on the market.
I will go for the 5D3. But not now. Hope the price comes down within the next 12 to 18 month. Saving up for a WA as well. Not the 16-35, the 17-40 is plenty for me as an amateur. As I already have a 77mm ND 10 filter, it doesn't make much sense. I do daylight longexpoures whenever I get down to the ocean...In the meantime my trusty 30D will do the job.