You keep harping on 4K video being a niche market. Yes, I agree that it is a niche market. But what you fail to see is how much money is inside that niche market. For example, let's compare the EOS 1D line to the "Rebel"/xxxD line, shall we? I'm sure the profit on the "Rebel" line is a gazillion times more than that on the 1D line, would you not agree? And the 1D line is also a very niche market of either true professionals or super-rich amateurs, not true? So, then, why does Canon keep the 1D line if it is made for such very niche market and the "Rebel" line is where most the money lies, huh? ... Oh, before you answer, please consider Leica.
It could be that they regard the 1DX and the "big whites" as prestige items, with the view that having these in the lineup improves sales of the lower items.... something like "Pros shoot with Canon, you should too". It is probably more to do with marketing than anything else. With video, when you go to prestige items, you end up with cinema cameras and lenses.
My opinion is that what is holding Canon back from 4K video is the ability to dump to storage at a reasonable speed. Until that problem is solved (CFast?) you are not going to see 4K video.