FF does not fit in an RX-1, that's why it has a fixed lens. The lens includes the optical path for the light to get to the FF sensor. LEICA = smart engineering and their FF body is the size of the 6D minus the viewfinder.
Your examples prove what I am saying and you are not understanding. Fixed lenses to accommodate the approximately 35mm or 1.38" optical path minimum required between the FF sensor (35mm film) and the last lens element. Add packaging for the body case and electronics and you are at around 1.75-2.00 inches minimum for a FF. Film was easier, you could pretty much have the film slide along the back of the camera, not so with a sensor.
I do understand. We are talking of mirrorless here. No need for a lot of space betwenn rear lens element and sensor plane.
Whether "the light path is built into" a non-interchangeable prime lens stuck unto the body or into an interchangeable lens mounted via a lens mount makes no difference whatsover. Except the first "solution" yielda a dumb, inflexible single focal length camera and the second solution yields a versatile camera-system. :-)
Flange back for Leica M for example is is 27.8mm. I am quite sure 20mm are also doable with some smart microlens array and proper lens-design. And if the rear lens elements would stick somewhat into the camera body, the camera could be as thin as an RX-1 and the lenses could be really slim as well.
And that's exactly what I want. And what the overwhelming majority of the market wants.
I will definitely NOT waste time and money moving from my current Canon APS-C DSLR plus lens assortment (EF-S, EF) to a Canon APS-C mirrorless plus new lens assortment (EF-M) to finally a Canon FF mirrorless ILC Camera ("EF-really right") with still another lens assortment. No way!
1. I want to skip purchase of another DSLR
2. I want to skip purhcasing any further APS-C cameras
3. I want to move right on to a compact, hi-performance FF mirrorless ILC with
* excellent sensor
* fast contrast+in-sensor-plane Phase-AF
* hi-end EVF
* full ergonomic controls [i.e. 2 wheels! ]
* at max. 1/3 of the cost of a Leica M system - so basically at the price of Sony RX-1
The first company to offer this, will be my next camera system vendor. :-)