This was done, a guy who used Polaroids to check his large format work when Polaroid stopped producing the large sizes (8x10 or maybe 20x24?) had a custom made back, it was only 10 megapixels and cost him $500,000 to have 2 made (a usable one and a backup) ... He only used it to "check" an image before using normal film to take the official shot...
So it probably wouldn't be affordable to produce a large sensor, the guy looked into it and decided that maybe 10 photographers in the world would spend that kind of money for a non-scanning back and even at $200,000 price tag the added phone support and technical support that would HAVE to come with it wouldn't be sustainable as a business model long term.
This is all my memory of reading an article, my numbers could be off slightly.
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