I view Robin as the DC Universe's primary symbol of youth, and therefore impossible to go back to once you've moved on to a new, more grown-up identity. That's how it worked for Dick and Jason, and how I think it will work for Tim and Stephanie. (Which doesn't mean Damian is the end of the Robin line, but that's another matter.)
Nightwing, on the other hand, is a symbol of the effort to forge an adult identity that incorporates the best lessons of one's mentors without their less desirable traits. And as such it would still be appropriate for Dick Grayson to take up the Nightwing role when he stops being Batman. He did it once before, after all, with just a short hiccup in which he turned in his costume. (And that actually had the benefit of bringing on a better costume.)
Of course, there's no guarantee that DC's editorial team is thinking along the same lines.