During the lead-time to current photokina the dSLR, the favourite product of the photography divisions of Canon and Nikon became serious competition from the same companies. The announcement of the Canon R and the Nikon Z systems of mirrorless camera systems may or may not predict the death of the mirror equipped cameras. The shape of the new mirrorless cameras is quite familiar and with a casual glance no one would see the difference between the current champions with mirror and the aspiring champions without mirror. Shortly after these announcements came the press release by Leica, Panasonic and Sigma that they will cooperate around the L-mount, designed by Leica for the SL, TL and CL systems. The strategy of the L-partners is identical to the one forged by Olympus with its FTH project.It may be unprecedented for Leica to intensify this type of cooperation, but it also shows the weakness. The proprietary mounts of Canon and Nikon are so popular that independent lens makers will copy as much as is possible to provide their users with compatible mounts. Canon and Nikon have extensive lens ranges themselves and in fact no one who is part of one of the closed ecosystems can complain about the scarcity of choices.
Leica has shot itself in the foot by insisting on very expensive lenses and their capacity to produce the range of lenses that are needed to become a large player on the market are limited. This situation was already the problem with the R system. Too less lenses and too expensive.
The cliché comments by every reviewer of a photographic system are related to the limited range of lenses. I always feel unhappy when reading such comments. I am aware that every manufacturer has to fill every perceived hole in the product range (this is the reason why Leica has now such a broad portfolio of camera systems). For an individual photographer who is not victim of the compulsive buying disorder (CBD), only a few lenses are needed. And when a system provides these, there is no reason to complain.
So the management of Leica realized that the sales of the L-mount systems were restricted to the small community of loyal Leica customers who feel that the archaic M-system or the vanished R-system have to be complemented by a new system of AF lenses.
Expensive body and cheaper lenses or cheaper body and expensive lenses: the choice becomes yours.
Panasonic is already strongly involved in the design and production of the SL system and its lenses and Sigma provided several lens units for the R-system that were placed in Leica mounts at the factory, here and there.

The mirrorless community stresses the fact that the new enlarged diameter of the lens mount and the shorter distance from flange to sensor surface (no one ever talks about film emulsions!) enables optical designers to produce lenses with better performance. It may be. The original Leica M-mount had a diameter of 43.9 mm and excluding the mechanical bayonet tabs had an effective diameter of much less. The diameter of the optical throat is of course the important paramater. The Leica M flange to surface is 27.8 mm. So the light rays have to pass through a narrow tunnel and then expand immediately to a wide array. Every strong deviation form the straight path will produce additional aberations and this is most compelling reason to widen the throat of the bayonet.The Leica M lenses are unique in combining small size with a narrow throat and high performance. This knowledge the competition does not have and therefore they have to use the other trick: wide throat and tele centric designs. The price is a huge size, comparable to what Leica now produces with the L-mount designs.
The effective diameter is much less than the inner diamater: canon has a good example: the R mount has a inner diameter of 54.5 mm, but te optical useable diameter is only 45.2 mm.
The L-mount has a diameter of 51.6 mm (Z-mount = 55 mm, S-mount = 53 mm, Leica R-mount = 49 mm). The flange distance to sensor is for the L-mount 20mm (Z-mount = 16mm,Canon R-mount = 20mm). The diameter is not the best criterium: the area within the circle is a better one. Then the difference between the L-mount and the Z-mount is -12%, hardly an important value.

The S3 has been announced with a pixel number of 64M. It is evident in the Leica line up that the flagship cannot have a lesser number that the SL or M series. It is now only a matter of time before the L and M series will be upgraded to the size of 35 to 40 Mp, leaving the Panasonic S1 with the 47 Mp sensor as the high end version. And the Leica Stores have a new kid on the block to sell. Of course there will be overhyped stories about the performance of the S1 with the Leica L-lenses. Leica claims to be a leader in the processing of the bayer layout to produce lifelike colours and this was the claimed weak point of Panasonic still cameras. With some technology transfer the problem might be gone.

Overall it is clear that Leica, once the manufacturer with an integrated vertical production column, is now evolving into a design center for products with components specified by the center, but manufactured elsewhere in the world. It is a mix of the Porsche design buro and the Apple design center.