Re: Kodak could make a shed full of money by....
35mm film is guided by sprockets, and kept in position and flat by a plate in the back - but creating a thin sensor usable in non-interchangeable backs is not easy, even more twenty-five years ago.
But most prosumer and pro level 35mm cameras had removable backs. Usually, you can change them with backs designed to store more film, backs to print date/time on film, backs adding timers for long exposures and time lapse images, and even the firsts data backs able to store each photo settings.
With these cameras, it was possible to provide a digital back, without the need to cram everything - including power - into the cartridge space, keep the sensor in the right place, and allowing controls on the back itself for sensor settings.
Some of the early digital SLRs were exactly created this way - some of them exactly by Kodak (its DCS serie), using Canon and Nikon bodies. They were bulky, and were later replaced by fully integrated DSLR - no longer made with Kodak.
Camera makers were of course more interested to sell new cameras than updating old ones. Some backs prototype were shown by third parties, but never reached the market. They were expensive, full frame sensor were yet to came, and made the camera bulkier.
Actually, those digital backs never had much general appeal, outside situations were they were a real advantages, and only the more expensive cameras could be easily adapted, cutting out most the market.
Almost every medium format and large format cameras had interchangeable backs, and these got digital ones - but they were very expensive, and some, especially for larger formats, were "scanning" ones (working like a scanner), usable only for very static subjects, albeit able to deliver very big resolutions.