I would likely have to agree. Although, at this point, I don't see why Canon WOULD continue to upgrade video features in their photo-intensive cameras simply because they appear to be segregating the video from photo into a new grouping of sorts.
I can think of two reasons. The first is that regardless of the cinema line, the market expects it. One of things that made the 5D2 so successful is the combination of photo and video features. The option NOT to buy a separate camera was and still is a HUGE deal to a lot of people. Both from a usability/convenience point if view and value wise. I am not so sure that saying to these costumers that while in '08 you could buy a great stills camera AND a video camera for 3K, now you have to pay 3K + 3,5 or 10K. Sure, the 5D3 will do video better than the 5D2, but will it improve it enough to be considered as great as the 2 was in '08?
Secondly, while Canon now has its cine line, Nikon doesn't. That means that Nikon will probably push the video capabilities in its new DSLRs to the max. Can Canon afford to play second fiddle to Nikon in the DSLR stills/video hybrid department? I'm not so sure that Canon can easily afford reviews that go something like "Canon X is just as good as Nikon Y in the stills department, but lags way behind it on the video side".
Oh I can certainly understand that. However, if such were the case, then why start a cinema line that would differentiate itself from the photo line? If they supply a 5dmkIII with incredible video features for $3k, then how will they sell any of their cinema line DSLR, let alone the C 300? If they instilled a clarity and quality that is on par with the C 300 in the 5dmkIII, then of course everyone would be buying the latter and saving 13 grand (Unless you REALLY needed the HD-SDI outputs and the built in ND filters). The market may demand it, but it would seem like a mistake for Canon to have bothered with a Cinema line if they were to keep their video DSLR revolution in motion with the momentum the unintentionally started it with. From a business standpoint, either the photo line will get limited, or the cinema line will prove to be somewhat fruitless. At least as best as I can for see it. Nonetheless, I could certainly be completely wrong, and Canon may be completely comfortable just offering more models with similar options. After all, that seems to work for the Auto industry to an extent.
As for Nikon, you make a very valid argument - one that could be precedent in Canon's decision to make their photo line equally competitive. Although, I don't believe Nikon has much of a video department dedication. I can't see them making too many leaps and bounds into the video world, but I can definitely be wrong there too. It would be exciting to see what the come up with, no doubt.