As a video producer, I'd ideally like a 1.4x or 1.5x crop to match the C200 I use as an A camera. Not a FF camera. I certainly dont care about 60fps, which the A camera does, but is probably used less than 1% of the time on it. Most important feature in a B camera for interviews etc for me would be reliable AF as I wouldn't be operating it a lot of the time. Which would rule out everyone but Canon at the moment. I'd also really like the colour science to match my A camera, so if I had an FS7 I'd think about an A7s and if I had an EVA1 I'd look at the GH5. But I don't feel the need for a B camera 90% of the time, and if I did I'd take one of the C100s or the 5d4 with me.If a pro video producer wants to buy a nice stills camera that will also serve as a backup video camera, however, he / she may be turned off by a 1.7x, jello ridden crop. Pro video producers were very excited about FF HD video when the 5DII enabled it 8 years ago and this same functionality with newer 4K video is what is lacking in Canon’s current generation of FF 6D/5D class cameras now. Not to mention, the video quality is just not that sharp or inspiring.
With FF 4K 60p, pro video producers (and many hobbyists) would buy these things en masse for the purposes I described above.
B cameras for production companies and indie filmmakers is not a mass market. It's a very small niche. Understanding things like this is why Canon sells lots of cameras.
People (including me) loved using the 5Dm2 for picture only work (music videos etc, not really documentaries) not just because of the large sensor, but because it made interchangable lens systems for video affordable for small independents. Suddenly you could use fast primes, TSEs, macros, superteles etc to create all kinds of images that previously required serious amounts of money (whether shooting 16/35mm film or something like the old Red One). And the low light performance (especially with an f1.2/f1.4 prime) blew away the EX1/HVX200 style cameras that we'd been using before then.
Looking back to the 5DMii, it had relatively soft 1080p video with horrible moire, hell, at launch you couldn't even change aperture in video mode, let alone record at 120fps, IBIS, 4K, DPAF etc. It was used on features, tv etc... And you think the R is going to be unuable as a B cam?
If you cant take inspiring images with just about any contemporary camera the problem isn't the camera. Sorry.