Except the symbol of Harvey is a lie.
Yes, but it's what the city needed. The truth would've destroyed their renewed faith in justice. Ultimately it isn't Batman or Harvey that matters. One person can't save a city. The people of the city had to choose to save themselves, to decide for themselves to lead a better life and stand up for their own ideals. Batman and Harvey were the symbols, the myths, that inspired them to do that. And the whole point of both these movies is that the myth is more important than the man. Because men are fallible and mortal, but myths can remain pure and eternal and continue to inspire people to better themselves.
Which makes me inclined to give credence to some speculation I've heard about The Dark Knight Rises
, the idea that it might involve
But why pin Dent's death on any real person? Why not say Dent was killed/kidnapped along with Gordon's family by a factious person who remains at large?
Because, as I said, the people of Gotham needed to be weaned off of Batman. Batman gave them hope and inspired them to start building a functional city again, but his methods, necessary as they were in the depths of the chaos and corruption, are themselves too lawless and rough to be appropriate in a healthy, stable society with a functional legal and judicial system. So the people had to renounce their old, rough-hewn symbol of hope (Batman) in favor of a more civilized one (Harvey). They needed to reject Batman's methods, to outgrow their need for him, in order to truly take responsibility for creating and preserving the society he wanted them to achieve.
It's kind of like the Operative in Serenity
. He did what he did in order to create a better world, but he understood that the dark methods he employed meant that he could never have a place in that world. It's not exactly like that, but there's a similarity.