I work in documentary TV and what I have seen in 5 years is exactly what you are talking about Andrew - a lower/mid budget set up would be C300 and 5Dmk3 and cinema cameras would be sony or arri to now FS7, A7S2 and then a cinema sony or arri. Canon have LOST a huuuge market with doc makers, everyone is pretty much using FS7s because they are more versatile and shoot beautiful images, they like the A7S2s because they are small, can chuck their EF lenses on it with speedbooster and shoot in crazy low light.
We had an in house C300 up until recently, but sold it due to it not being used in a year. We also have a couple of 5Dmk3s and they only get used for stills these days.
I remember seeing the 1DC on a shoot and thinking how incredible the footage and that this may be how all their cameras would go... but... nope
I think they got greedy and wanted to push everyone on to their C-1/2/300s, they could have cornered the market! just imagine a 5DC and 1DC line running alongside the still camera lines with all the potential magic lantern unlock .. that massive error twinned with disappointing stills cameras being produced, can't see them keeping their dominance for much longer