It would have been too pedestrian and too depressing if they’d ended it with Rachel’s death and Dent lying in the hospital bed. You need to see what he does as Two-Face. The whole film builds up to it. For them to go ‘see you next time’ at that point would have been disasterous IMO. Even though everything HAS gone to shit by the end of The Dark Knight you don’t want the audience coming out at the end feeling the same way. The idea of the police chasing Batman is exhilirating, and you’ve got Dent being setup as martyr and that actually having a positive effect on Gotham.