Is the 5D III selling that much worse? From what I understand they're both selling very well. The D800 does look particularly great for stills.
I'm just not sure Canon will lose that many customers. And lens sales matter more for profitability than do body sales (presumably, most D800 buyers are upgrading from D700s, not 5D Mark IIs). As for video, the non-standard lenses available for the GH2 and black magic camera and their non-standard workflows put them in high-end enthusiast owner/op territory exclusively (and you really need to buy special-purpose lenses around video, pretty exclusively, for either camera, which is a significant expense not mitigated by versatility). And high-end enthusiasts isn't as big or lucrative a market as say rentals, wedding videographers, journalists who want to shoot video and take stills, more casual hobbyists, etc. Most hobbyists (the biggest market by far) are interested in ease of use, which is the real advantage with Canon cameras, that and lens availability (just look at any rental shop's inventory); most professionals are interested in reliability and compatibility over image quality, and the Canons and Nikons excel there. There are still huge markets for which "good enough" but easy is great.
The 5D III is a pretty boring jack-of-all trades camera, but for a professional shooter or hobbyist (both of whom want versatility) it does everything well enough and the Canons are exceptionally reliable. It fixes aliasing and bad AF, the two most common complaints with its predecessor. I don't think Canon will gain any market share this generation, but they won't lose much either. Canon's biggest miscalculation was making the 5D III so incrementally better than its predecessor that most shooters (stills or video) won't upgrade, but Mark II owners still buy Canon glass…. If the C300 does fail, that might be more of a legitimate wake up call than the 5D III's middling success, but the progeny of that are years off, at least...
The D800 does look great though, doesn't it?