I think that Nikon, like other major manufacturers, are now well past the phase where they expect the majority of users to upgrade with every new release.
Nowadays, they compete on "features" and will constantly try to leapfrog each-other in the quest for new users. After all, if someone is buying their very first camera (particularly a system camera - either dSLR or CSC), they have no brand loyalty or lens tie-in at that stage and will opt for whichever seems best specced at the time.
But for existing users, there is really only a need to upgrade when a fault, flaw or inadequacy in the existing product has been corrected or improved significantly.
The market would seem to suggest (in UK at least) that most users upgrade to a higher-specced model rather than an upgraded version of the same model. For example D3100 to D5200 or D7100 rather than to D3300.