Ive said much the same about the sales ceiling/floor.
However, although it makes business sense for DC to just ignore the book because it'll sell the same no matter what, I do believe that with a little effort and patience, Nightwing could break through that ceiling into a higher sales bracket. And since the floor remains so stable, if DC could get him there I suspect he'd stay there with the same consistency he shows now.
It'd just take a writer who wants to do it, and DC letting him/her do their own thing instead of derailing the title for Bruce's benefit.