Every time somebody comes up with the "new" idea of making a digital back for old but popular SLR cameras, like the Nikon F3 or Canon F1, people who claim to know are very quick to cut the idea down. Too expensive. Too complicated. There isn't room enough. Etc. Etc. Etc.
But Kodak did just that. They did it in 1988 with 1988 technology, and it worked. But not now.
I'm not sure if I care, but if somebody did this and made it available for (insert name of great, old camera body here), I just might be tempted enough to buy one
Electro-Optic Camera: The first DSLR