I would fit in the target market. I own a Nikon F2, Nikon F3, Nikkormat EL, Nikon FM, and a Nikon FM2n, along with a bunch of lenses for them. I really like the ergonomics of the old manual focus Nikons. The viewfinders are far superior to any DSLR I've seen, and the controls are completely intuitive, with no menus, mode settings, or other computerized automated stuff to get in the way and mislead you. If you want to change the shutter speed, turn the shutter speed dial. Change the aperture? Rotate the aperture ring. Change the focus? Rotate the focus ring. Each control has one function, and each function has one control.
But I bought a Nikon D200 DSLR about 6 years ago, and have only shot a couple of rolls of film since. The D200 isn't as nice as the F3, but it's nice enough. It lets me use my old lenses. And it has a preview screen on the back that lets me see that I've captured a shot. Its viewfinder, while not as big and bright as my F3's, does have the nice property that it shows me exactly what my sensor is going to capture, unlike what would happen with a crop sensor used on a standard sized old 35mm SLR.
There are only so many old 35mm SLR bodies hiding away in closets, and the supply is unlikely to ever increase substantially. So the market for a digital adapter is likely to drop off to near zero once everyone who wants one has it.
A large part of the joy in using the old mechanical 35mm cameras is in the big, bright viewfinder. But unless the digital sensor was a full 24x36mm size, much of the viewfinder would be wasted with a digital back. And a "full frame" sensor would be frightfully expensive.
There was never any standardization of the distance between the film cartridge and the gate where the image was formed. Any "digital film" type thing will have to have adjustability in order to fit a variety of cameras. Combined with the need to fit into the very thin space of a piece of film, I think the "digital film cartridge" idea is doomed technically.
A digital film back, as discussed in chicken's "economical" link above, is more viable, but still not viable enough, for the reasons outlined in that link.
I wish I could use my old cameras digitally. But realistically, it's not going to happen. Any digital adapter would have a limited market, and would have to compete with the concept of a completely integrated DSLR that could use my old lenses.