Hasselblad did that many years ago for their cameras (still do, I believe). I have seen several attempts by companies to retrofit various imagers (usually CCD or CMOS, mostly the latter these days) but am not sure any really worked out well. B&H Photo probably has a selection. But that idea was much more popular when digital first came out; now that it is mainstream (and then some) and has been for many years, I don't think there's enough of a market for retrofitting classic cameras (which are themselves wearing out).
I tend to agree with
@restorer-john; whenever I have thought about doing something with one of my older cameras, I found either cheap junk (in the "affordable" for me category) or unwieldy, expensive behemoths. Lots of extra weight and more seals to let in dust and moisture. I think the market just is not there for the inexpensive quality replacement back; consumers buy inexpensive digital units, and pros expensive digital units, skipping the retrofitting. Ultimately I just bought a digital camera and my old film cameras sit in their bags awaiting a museum -- or estate sale.
I actually have owned several of the mentioned digital backs from Hasselblad and Phase One. The first one I had was made by Imacon, before Hasselblad acquired it. It was a cumbersome solution with an external hard disk attached to it. It was 16mp with 64mp in multi-shot mode. The result was stunning, of the standard of that period of time, of course. I remember at that time almost no digital cameras could produce sharp enough image at 100% view. There was always a kind of softness (mainly due to low-pass filter). When I viewed the first file coming from the digital back, it was really a shock. The color and details were way better than other digital cameras / DSLR. Kodak sensors really made good color.
It was quite funny that at that time, most debates were still about film vs digital. People had heated debates over and over again on whether digital images had a "film character" - almost a magical quality that impossible to describe. Pretty much like the aura of LP I suppose. And CCD was regarded as more film-like than CMOS. For a long period of time, I personally also preferred CCD to CMOS. CCD did have better color at that time (though worse ISO performance). And for the later digital backs I had already didn't need an external hard disk. There were just like a 120 film magazines. I think they were pretty good solutions. I was happy with them for fairly long period of time.
The problem really came when the resolution went beyond 50mp. I just couldn't get my Hasselblad CFV-50C CMOS back to produce sharp images handheld. Hasselblad V system is a very, very old system. It simply could'n't afford the precision needed for 50mp, also the mechanical vibration. I already had my camera body, the prism finder, focusing screens all calibrated. But it is still very difficult to have a good rate of success in sharpness.
But on the other hand, for autofocus systems, I think digital backs still work. I still like my Phase One + Hasselblad H combo. With the use of True Focus, AF works. Everything is still great. Honestly, it is much better than many so called sophisticated AF system. But I agree that to dream about attaching a digital back to old camera systems is pretty much a hopeless project. Even if the digital back can go cheap enough, the old mechanical cameras most of the time just can't catch up with the need of the digital.
