And that's it. If you can't understand this point, you probably don't need to be watching this movie.

it's not a question of "understanding this point", it's a question of agreeing with the scope of the point. neither gordon nor bruce would mortgage batman's image for a dead hero - there's only so much inspiration a dead d.a. can offer, whereas batman's role as "providing an example" for the citizens of gotham is going to LONG outlast the memory of one dead d.a. - remember what happened to harvey's predecessor. besides, since he was dead, without direct proof linking him specifically to any crimes, the crimes themselves would remain unsolved - and unsolvable, and his legacy would remain untarnished without linking those crimes to anyone.
if harvey was still alive, though, and potentially prosecutable, or at least investiatable, his reputation stood to be tarnished, his former prosecutions compromised. this required batman to temporarily take the fall for these things. when gotham is ready, someone - possibly the riddler, if not harvey himself - will "out" 2face as the real criminal, and in the process batman will be redeemed in the public eye.
harvey's alive, and he'll either return as twoface, or he'll get "healed' like he has on a few occasions in the original stories, returning to his role as harvey dent, white knight, but with the two-face persona as a constant, looming threat, ready to rear it's half-head whenever life gets snarky on him.
and lastly, as for understanding batman, harvey and the joker, i read my first harvey/joker batman story some time around 1979, if not earlier.

if there's anything i've -ever-understood in life it's comics.
