I agree, it is really dissapointing that there will never be a c100 mark iii. I thought that there was supposed to be a ILC XC20, which seemed likely for a long time and now, it's looking like if they ever do that camera, it wil not be an interchangeable lens. Which, is really kinda pointless in my opinion to spend that kind of money (the xc15 is still close to 2k$) on a camera that you cannot swap the lens...While it's disappointing to hear that a C100 III isn't likely, I can understand Canon's reasoning. The Panasonic S1H, upcoming Sony A7S III, and possibly even the Sigma fp would likely outclass it while sporting full-frame sensors at lower or equal price points. The C200 appears to be the new entry model for the cinema line.
A non-linear successor to the C100 line that I'd like to see is an ILC RF mount XC camera with a 6k Super35 sensor. It doesn't need Cinema RAW light like the C200, just a decent 60fps of uncropped 6k with the rock-solid C-log profiles and 10-bit 4:2:2 internal. I think a camera like that would put up a hell of a fight against the S1H and A7S III even with a Super35 on a full-frame lens platform.
It looks to me they are forcing the EOS R as a response, which I finally gave in and ordered one, so I cannot say that they are completely wrong.