Leica has no other choice than this new MF system.
Current 35mm reflex systems are very versatile. You must to offer many lenses, from extreme wide-angles to superteles, macro, tilt and shift lenses, portrait lenses, zooms... etc... The costs of manufacture and distribution of such a system are very high. You need a large stock of all your lenses and accesories all over the world, in the hands of many retailers. On the other hand, Leica cannot compete against Canon, Nikon, Sony, etc in terms of image quality on the same format. They cannot. The digital photography is determined by the lens, but also by the sensor, the processor, the software... Leica cannot be competitive in terms of price either, due to economies of scale. So, how can they survive in the 35mm market in the long term? It is impossible.
They need new customers, and a different product. A few lenses for specific (professional) tasks (landscape and studio work), a small camera (much smaller than 645 and 6x6 based systems) with great image quality (better than 35mm at low ISOs). This product will be competitive against 35mm (for particular types of work, due to image quality), and competitive against 645 format (much smaller body and lenses, less expensive).
I don't know if this strategy will be a success, but it is the only product concept with some possibility of success in the long term for Leica.
I don't know about the compatibility of actual R lenses, but the mount-to-sensor distance of medium format cameras is much larger than that of 35mm cameras. The R system is 47mm, and medium format is around double of that number. If Leica does without the mirror box (like the micro 4/3 system), they could keep the register of the R system untouched.
All MF systems are based on 645 (Hasselblad, Mamiya) or even 6x6 format (Rollei/Sinar). Hassel will go full 645 next year. Meanwhile, they have 36x48mm. The same goes for Mamiya/Phase One, but they hit first: full 645 sensors, for 645 lenses. This equipment is really big.
The Mamiya ZD was a good try, but it is a 645 camera with a cropped sensor (even then, it was similar in body size to a 1Ds Canon). The new Leica will be even smaller. A good example (and unique) of "optimized for smaller than 645" is the Hasselblad HCD 28mm. From this only case it is difficult to get an idea of what can be done for something like 36x48, or a bit smaller format.
The cost of the new Leica MF system cannot be the same of Hasselblad or Mamiya's, or Leica will have problems.