Dx and Fx each have their virtues and have co-existed in the Canon line for years yet as I understand it, FF DSLR's are still less than 10% of their line. I do agree that the cost of sensors will come down in accord with Moore's law like every other chip has doen but I cannot see any reason for Dx to fade away over time. For wildlife shooters in the Canon camp their Id Mark III has a 1.6 crop and is only 10mp and the DSLR's of choice.
As for what Nikon plans are for Dx lenses I do not know. Dx lenses are only needed for the wide end with FL starting at less than 24mm to be more telecentric for the microlenses in the sensor. Computer aided designs of lenses are allowing significant improvements in the optical fomulas of only a few years ago plus we now has nano-coating. It simple make cents (and dollars or yen) for Nikon to replace these lenses for the simple reason that people will buy them and Nikon will make money. Nikon's 12-24 is the oldest design for a superzoom. Nikon's 17-55 is a great lens but really needs nano coating since my only complaint is flare. If Nikon can engineer a 17-70 f/2.8 DX it would certainly sell since 70mm is a good portrait length for DX. I now use my 35-70 f/2.8 for that.
I don't know what they are thinking or will do in the years to come but so far, when they build it, NAS kicks in the people buy it.
Albert J Valentino Nikonian Moderator Emeritus Vantage Point Images Mastery of Composition is the Key to Great Photography