The Nokton 50f1.5 was one of the first "cutting edge" lenses that Mr Kobayashi released 10 years ago (the 15f4.5 being another one). The Nokton is also the first 35 format lens with two aspherical surfaces on the same element! It was and still is one of the best 50 mm you can find - and for the price - one of the great bargains of all time for Rf users.
The policy of going to M-mount seems to be that the market for straight screw-mount lenses is more or less filled. There are a lot of "shooters" using old LTM bodies - but I suspect that most of them now have their stock of LTM lenses, be it vintage Leica or current VC lenses.
The M-mount reduces the problem (not a big problem, mind you) of users having to source M-adapters and as these are of varying quality, particularly 3rd party made one - the M-mount offers consistency in the mount and back-focus.
Mr Kobayashi and his design staff also looks ahead. Making the same thing for 10 years is a bit boring and some improvements can be made over time.
One of the limitations of the LTM mount is the size of the rear "pupil" or element. It is limited in how big a diameter you can make it. With the M-mount you can increase it with a couple of mm - which in optical design is a big step. Case in point, the 35f1.2 Nokton - when I asked if it could be made as a screw-mount - I was told that the thread would have to be cut in the glass of the rear element and nobody thought that was a good idea!!!!
