There's a segment of the market that will slowly migrate to 4K TVs and next-gen disc systems. It's true that Canon doesn't need to rush to reach these people. But again, these people are only a part of the market-- and they're not necessarily the most important (e.g. trend-setting and profitable) parts.
A lot of 4K content will be streamed rather than viewed via hard media, such as discs. Likewise, a lot of 4K content will be consumed via computers and tablets, not just TVs. I see a lot of studies on different trends in device usage, and I spend a lot of time talking to the people making the next generation of devices. Canon doesn't need 4K today, right this moment-- that's true. And for some customers, Canon might never need 4K. But within a year, if Canon doesn't offer 4K at a relatively accessible price point (e.g. $3-5k), it will do so at its own peril. 4K will be too relevant, and Canon's restrained video implementations will be look too soft compared to the competition.
It's also worth pointing out that 4K is useful even if your final output will only be 1080p, as others have mentioned. Canon was happy to talk about how great the 5D Mark III was for media pros who need both stills and videos. Well, I work in that world-- and 4K would be nice. Re-framing the image, getting sharper 1080p, having options as screen density increases-- these are all legitimate uses.
Again I see the market for 4K on a 5D body being potentially greater than 4K on a camera like the C100. The latter still isn't cheap and I would image that the TV and film markets that care about 4K use are going to be using the C300/C500.
The kind of use video on a 5D body gets on the other hand seems more likely to benefit from 4K. I would imagine for example that couples getting married may well care about "future proofing" video there wedding shooter may take plus as has been mentioned 4K makes editing shots you don't have much time/room to setup easier. With amateurs your likely dealing with people more likely to buy into 4K earlier than the general population plus again more likely to desire the extra editing latitude.
Not being a video shooter I would imagine that the functionality of the C100 is far beyond the 5D so the former could probably still find a market even if the latter offered 4K.