When I had a Nikon D100 about nine years ago I wondered why no one could make an adaptor to make the field of view the same as would be on 35mm film.
The answer is that they could but the lens would have to move closer to the sensor so it was not a viable option when using mating lens mounts.
However, using a lens of one manufacturer on the body of another which is significantly slimmer gets over this problem. I know it's a digital product which we are not supposed to discuss here but it's the optical design which is of interest. I think the company should be congratulated for pursuing it.
This is actually an idea which a camera manufacturer could incorporate into the camera rather than as an adaptor to give full frame coverage with existing lenses. I'm surprised it hasn't appeared already. I think a camera with an APS size sensor which could take e.g. Pentax K or Nikon F lenses (manual or auto-focus) and give full frame coverage would be very popular.