Again the misconception of "bad quality just below x milimeters".
No, it's not a misconception. There's an old saying (originally from boxing, but borrowed early in the 20th century for photography) that "a good big 'un will always beat a good little 'un." For technical quality, this is undeniable. It's not a question of bad quality, but of poorer quality. You can't really dispute this when talking about technical quality.
Aesthetics are another matter, and different cameras encourage different ways of working. You can get pictures with a Leica that you can't get with an 8x10, and that you could only get with difficulty on roll film. But there's nothing you could shoot on a half-frame, or a small format digital rangefinder, that you can't get, easier, with an existing full-frame 35mm camera or M8/M8.2.
Smaller formats also bring diminishing returns. A half-frame camera can't be half the size of full-frame, especially if you want a coupled rangefinder: the finder itself just isn't going to get much smaller than what you find in a CL, for example. This is why a Retina IIIc/C is a lot bigger than a IIa. The controls have to be big enough to use, and the markings big enough to read: don't forget how fiddly the Minox 35 was.
I can see at least two possible reasons why the camera you describe doesn't exist. One is because you probably can't make a metered, manual-focus, interchangeable-lens camera, with a good (brightline, parallax adjusted) finder in any format, film or digital, much smaller than a Leica CL. The other is that even if it were possible, the demand would be very low indeed in a world where you can buy second-hand CLs, screw-mount Leicas, Retinas and so forth.
Cheers,
Roger