I get that, but Robin =/= Batman, and it was pretty heavily implied that Batman could be anyone and could essentially live forever. Why not just be John Blake? Why does he have to be Robin?
It's a nod to the fans who have wanted Robin in the series. That's all. A send off for the Batman trilogy an attempt to mix comic mythos up and still appeal to fans who want to scream, "BUT WE NEVER SEEN ROBIN!!!". And seeing how Robin has taken over the mantle before, it's not *That* big of a stretch.

