Wow, I think that's getting it completely backward. He didn't become better -- he became obsolete. That was the thrust of the first two films. The idea was that Batman was a stopgap -- an imperfect but necessary emergency measure to bring hope back to a totally broken and corrupt city, but not a real solution in the long term. The idea in The Dark Knight was that the people and government of Gotham were rebuilding to the point that they could take care of law and justice for themselves, so that Batman was no longer needed. Bruce saw Harvey as a better symbol than Batman, because he was a symbol of real justice and functional social order, rather than vigilante justice as a check against chaos. And when the people of Gotham proved that they would stand up for what was right rather than giving into the Joker's mind games, that proved that they could take responsibility for their own law and order, that they were a functional society again and Batman had become unnecessary, except as a symbol of the lawlessness they had now transcended. That was what Gordon's closing line meant -- the people needed to stop seeing Batman as a hero, to outgrow their dependence on him, in order to grow up and take responsibility for themselves.
