-I don't really care about 67 AF as I rarely use it now on a Canon and don't have it on any of my MF or LF cameras.
I'm with you. But the market for manual-focus-only is simply much smaller than auto-focus. Smaller market = higher required markups, due to less economy of scale.
-capture rate again, wouldn't it be just twice as long as the existing sensors now that are half the size?
Would depend on the design/resolution. It wouldn't necessarily be that much of a speed hit. Actually come to think of it I don't think this would be a major issue.
-as far as a the economics go, yes there might be some but it would be a limited camera and priced as so.
This sort of reminds me of an art director who expects a minimilist-style image will be drastically less expensive/time-consuming to produce than an ornate style. This simply isn't the case. Even eliminating all bells and whistles any new digital back requires tremendous R+D. With a market even smaller than the general medium format market the cost-per-back of that R+D would be very very high.
As reference there was one commercial attempt at a 6x6 back. The "BigShot" - it was very expensive and a pretty big flop.
Remember I'm not looking for some crazy spec on the [6x7] sensor, just what already exists for low light
In a very fundamental way, based on the way sensors are designed and fabricated the size itself is a crazy spec. Not impossible. But as pointed out, very difficult to see where the economies of the product would work.
21mpx4=84mp=too large of files to edit in reality
As a side note I edit 80mp raw files on a regular basis on a 4 year old laptop, alongside 16-22mp dSLR files; it's really not much slower. In the case that I don't need the resolution for the particular case I process e.g. a 50% resolution TIFF which keeps the retouching workflow snappy. When I need the resolution the 16 bit TIFF is a rather hefty file to retouch, but by definition it's worth it (if it wasn't worth it for a particular job I simply process a smaller TIFF from the full sized raw).
Until you've got a few hundred 80mp raw files on your computer I can completely understand where you'd think they would be insanely huge in practice. That's simply not the way Capture One works. Editing 80mp raw files is simply not 4 times harder/time-consuming than editing 20mp raw files.
I shoot mostly Canon digital but find myself shooting more and more 6x7 film again and scanning for the dof and image quality.
Yes, I'm putting myself out there on this and all you pixel peeps will start taking your shots but if you have ever shot 6x6, 6x7, 6x8 etc you will know/see the difference.
Doug if I had the money for an IQ160 or Credo and a fast lens, I probably still wouldn't be happy with the dof since it is still just over half the size of 6x7.
I don't think I'd count as one of those pixel peepers. I have great respect for the non measurable characteristics of cameras/lenses like the look and feel of the image, the color rending (sometimes accurate and pleasing color are diametrically opposed), and the balance of the body, ergonomics of shooting, and other non tangible/provable things about a camera system that increase the feeling of that camera functioning as an extension of your will rather than an obstacle to it.
From your writing it's not 100% clear to me if you've ever done any major shooting with 645 MFD, but it sounds like you have not. If not I'd really
encourage you to try a kit like a P65+ with the Phase 150/2.8, or Hassy 100/2.2 or Hassy FE 110/2 before you assume that nothing smaller than 6x7 would make you happy. You know what they say about assumptions :-). You have your choice of 15mp, 20mp, 60mp, and 80mp 645 sensors* A Phase One 150/2.8 lens and a P65+ or IQ160/180 produces aesthetically beautiful narrow-DOF rendering with great IQ characteristics.
Of course you may try it and not find what you want. But you threw out a price of $20k for your imagined 6x7 back and I can tell you there are some great kits at that price already. They are not 6x7, but instead of assuming a specific numerical technical spec (6.0 cm by 7.0 cm) is what you need to achieve your vision I'd suggest stating your goals (e.g. beautiful look, shallow DOF, good transitions in and out of the plane of focus, clean rendering) and see if something will achieve that within your budget.
And re: the low-light element the P65+ looks very good at ISO1600 in sensor+ mode (IMO of course - I'd be happy to send you an example raw for you evaluate for your own needs/standards). Since you said resolution is not that important to you I think that would be a great option for you.
Unless of course this is an armchair discussion and you have no real intention/interest in such a camera beyond ideal conversation
*The 15mp and 20mp refer to Sensor+ with an IQ160/P65+ or IQ180 respectively