To start, I'm going to assume that Morrison writes well enough that no character other than Nightwing or Robin will replace Batman, since his run spotlights none of the other candidates. Yes, I understand that Morrison isn't writing Battle for the Cowl, but I presume his run exerts major influence on its story because Batman has been the flagship since Morrison came aboard. So for all of those out there crying, "It's Jason Todd because he wears black and red!" or "It's Harvey Dent because in a story Morrison never read Harvey Dent protected Gotham City!" you're wrong and you've mistaken Morrison with a hack.
With that in mind, note that Tim receives a lot more facetime than Dick in Morrison's run. Tim deals with Damian. Tim battles Bane-Bats. Tim endures the trials on Mayhew's Island. Tim "saves the city" from the Club of Villains in 681, while Dick froths at the mouth waiting for his lobotomy. Overall, Morrison has established a stronger presence for Tim and, in my view, writes him as a better superhero, one more deserving of the Bat mantle.
What's more, the theme of father and son umbrellas Morrison's Batman run. Note, for example, the arrival of Damian or Dr. Hurt's many allusions to Thomas Wayne or Cyril Sheldrake inheriting his father's calling as the Knight. While one might argue that both Dick and Tim serve as surrogate sons to Bruce Wayne, in reality, the age gap between Dick and Bruce isn't all that wide and Dick doesn't interact with Bruce the way a son does with his father. On the other hand, Bruce possesses legal guardianship of Tim, who is an appropriate age to be his son and pretty much considers himself as such. Note that he calls Damian his brother in 676.
Additionally, Robin pulls some very distinct Batman-isms over the course of these issues. For instance, Robin admits to a complete dearth of personal life in 667: "My choices? A weekend nursing my should and dodging Alfred... or a trip to a private island to hang out with a bunch of C-list crime fighters... I'm totally sad either way, Bruce." Interestingly, he says this as Bruce ventures away from that lifestyle, visiting old friends (The Club) and building a serious relationship with Jezebel. Later in that arc, Tim spots an out-of-place light source which leads him to a secret passage, a very subtle clue similar to the chicken grease clue Batman uncovered earlier in the issue.
I admit, the previous two examples only illustrate Tim's growth into a more dedicated crime fighter, more like Batman, but the next two examples speak volumes to how much Tim is actually turning into Batman. In Batman 655, Bruce Wayne keeps forgetting to drop the Batman growl in his voice when chatting with his socialite friends. In tandem with this, we see Robin forget to remove his domino mask before cycling out to town in his civies. Fortunately, Alfred stops him before he leaves, but it's a very Batman-esque blunder that betrays a difficulty in separating himself from his superhero persona. Morrison also seeds a strong clue in a seemingly throwaway scene in 675. In that scene, Tim out-Batmans Dick by more accurately predicting the time it will take for "Ray-Gun Rider" to fly overhead; he calls it at exactly 2.75 seconds. This sort of obsession with exact detail, to have the timing down to the correct hundredth of a second, echoes Batman speech to the treacherous monk in Nanda Parbat and his Black Casebook captions before he emerges from the grave in 681. Note also that 675 is the ONLY time we see Robin and Nightwing together in all of Morrison's run, so Dick never evens the score.
Let's journey outside the text for a second now. The strength of the Dick Grayson/Nightwing character carries two books: Nightwing and Titans. The strength of the Tim Drake/Robin character also carries two books: Robin and Teen Titans. Currently, the Dick Grayson books outsell their analogous Tim Drake books with Nightwing leading Robin by a substantial margin. People will read Batman no matter what, especially with a new creative direction. Just like the wild success of Brand New Day, intrigue always beats outrage, as does collectors' completionism. So it makes a lot more sense from a sales standpoint to resume Dick Grayson as Nightwing, relaunch with a new #1 to thrust its already-high sales further out into the stratosphere, and just ditch the Robin book altogether or perhaps restart it with Damian in the title role *shudder*.
This brings me to another point. It's fairly obvious from Tony Daniel's drawing of "Batman and Robin will never die!" that Damian becomes Robin. If Nightwing replaces Batman, why would he recruit Damian, someone he's never met, as his Robin when he could just as easily have, well, ROBIN as his Robin? It would make no sense. Also, I really doubt DC will go with two Robins, so the writers must adopt one of three alternatives.
- Bump off Tim Drake - No.
- Retire Tim Drake - Hasn't been set up at all, plus Tim is awfully young to retire.
- Transform Tim Drake into a new Nightwing - Feasible, however it leaves a nasty taste in the mouth, which I hope DC would recognize before proceeding in this direction. Having Robin become Nightwing again not only rehashes an old storyline, it also contrives a very mechanical shifting where everyone is driven up one slot in the hierarchy, an RPG style level-up for the whole party, not exactly germane to organic storytelling.
But Tim became Robin exactly because he believed in the Dynamic Duo. In Tim Drake's eyes, Batman needs Robin and Batman and Robin must "never die."
As these excerpts imply, Tim Drake's introduction in A Lonely Place of Dying teems with speculation about the mythic resonance of Batman and Robin.
Now if the above didn't sell you on my theory, then how about this? In a future scenario penned by the God of All Comics himself in JLA #8, Tim Drake takes over for Batman with Bruce Wayne's son as his sidekick! Enlarge the picture to read that lower-left caption box.
Still not convinced yet? Of course not. That story's 10 years old. Let's go a little more contemporary, like Batman 681. This one is the smoking gun that case-closes this mystery as far as I'm concerned. On page 7 of Batman 681, the late Dark Ranger's sidekick the Scout dons the costume of his fallen mentor and zaps the Swagman. Obviously, this foreshadows a whole student-becoming-the-master type deal to occur with Batman. Fine, this could implicate either Robin or Nightwing. But notice, on the very next page, Robin's face appears reflected in the glass of the new Dark Ranger's helmet, symbolically linking the two, singling out Tim as the cowl's inheritor. Sure, later on in the issue we get a pinup of Nightwing brooding over the cowl, but this is Morrison we're talking about. With him, the safe money's on subtlety.
EDIT: Hey if you enjoyed reading this page, please check out my annotations on Morrison's Batman at morrisonbatman.blogspot.com. Thanks!