It's expensive to develop a new line of lenses, and I would expect this to be especially difficult after R&D cuts. Also then they risk losing their F mount customers as Sony did with A mount. Mirrorless buyers would wait for new higher end lenses in the native mount instead of upgrading to F mount lenses and using an adapter. So Nikon's existing lens sales would die off as people would hold off purchases. I think it would be better if each camera company specialize on something and do it well, instead of diluting their resources on a lot of different product lines. Perhaps Nikon will indeed make yet another mount. If they can now develop 5 lenses per average year, and if we divide that between three mounts (1, mirrorless large sensor, and FX) then each of the three lens systems will progress very slowly.