Then, the market for video is much bigger than the market for MF photography. The video market would be large enough to finance production of stepper technology for making large CMOS sensors at a more reasonable price.
Not at all clear that that's the case to me.
The market for large sensor video, for example, is so small that Arri had to cannibalise Hasselblad lenses to make lenses for the Arri 65.
http://arrirentalgroup.com/alexa65/Cine lenses are so damned expensive precisely because the market for high end video is actually a lot smaller than high-end (ie MF) stills. Sure, the mechanical and optical requirements are more demanding too but the main thing is that they are produced in TINY production runs. Sony built new lens-making machines for GM series stills lenses- not cine lenses.
4K for acquisition has definitely made it and is here to stay. There's a very strong case for acquiring in 4K even if delivering in full HD or 2K.
In fact lots of feature films shown in cinemas are finished in 2K (even if they were shot in 4K or more). We don't have much in the way of 4K delivery infrastructure yet, even if we have sensors and displays capable of delivering it we just don't have the ways to get it out to people (except in ultra-compressed online forms, which I'm skeptical really deserve the 4K label if they are throwing away 99+% of the image information).
At typical viewing distances in the home for human eyeballs, 4K has marginal gains over 2K:
http://prolost.com/blog/2013/1/22/4k-in-the-home.htmlIf you've got a projector and a big screen, it's worth it, but not otherwise. Contrast and black levels are much more important to perceived quality at typical viewing distance.
Computer monitors are higher resolution, but it's not clear how many people actually use them for media consumption rather than media creation. They are more likely to use iPads etc, which whilst in theory are capable of vast resolution with retina screens actually look perfectly super with full HD video. Same as televisions, and even most projectors in the home, as above.
All of which means that whilst I can see 8K video as desirable for acquisition, and probably inevitable (RED and Arri are around 6K already) I don't see that that necessarily means it will trickle down to consumer level gear any time soon.
It might, because of the numbers game to sell us cameras. But broadcasters and film-makers are a small market, not a mass market.
Cheers, Hywel