heh, it's going to remain a possibility with every third party products and it's a valid reason for their accordingly lower (in general) prices :-)
Most possibly Nikon doesn't see a better way to make things more difficult for a competitor who delivers better lenses lately than the genuine ones. Old games/habits never die.
Do you realize Sigma is offering a free firmware upgrade service to the old lenses? I wonder how good the old Nikon lenses will work on that body.
I don't know. You need to update the firmware of fuji lenses for the xe-2. I'm not saying nikon change is deliberate but I simply do not know enough about the df and how or if it had legit changes in the coupling.
Having said that - you used to buy sigma because it was cheaper; now adays folks buy sigma because they are better. I really think people should give sigma a thumb up for good quality and good service (Klaus has hall fo shame but what about a hall of fame?)
I mentioned Sigma's turnaround in the recent lens tests ...