So what I'm gathering from all of these replies (which, as the OP, I appreciate very much), is that there is no obvious successor to the GNX and that the next greatest thing that will compel us to switch has not yet been released.
The only thing I would add to that is that if there's something you consider a major deficiency of the Nexus that's addressed in an otherwise equal device, that might be a reason to upgrade. In my case I really miss the brightness of the display on my Bionic. I often have difficulty seeing my Nexus screen outdoors, but this was NEVER an issue with the Bionic.
But I digress, TECHNICALLY we are all still able to upgrade with discounted pricing as of August 15, 2013 at the 20 month mark. Although this will be the last time for that.
This Nexus from Samsung has really burned me...don't really see me owning another Samsung smartphone. The radio/reception on this thing has been near crap...to put it mildly. One thing you have to give Motorola they probably have the best radio/reception in smartphones. Even thought Samsung has the S3 and 4 and the Note 2, I still can't see myself owning another Samsung.
But $200 is a lot easier to swallow than 600-800 up front.