Ack! -- this sounds like my English class.
As to the theme which is the subject of this thread though, I think that it really goes without saying that Sauron's destruction will not represent an end to all evil forever, and that Galadriel's words can be interpreted accordingly. Although the external "personification" of evil has been defeated, it stands to reason (to my mind at least) that this will not mark an end to the internal evil within the hearts of Men (and the other races). Galadriel's words, to me, did not entail the whole annihilation of evil entirely but simply the evil of Sauron.
Evil for Tolkien was intimately connected with the desire for Power, Domination, and control over one's own creation. This is a psychological appreciation of evil as something we are all capable of feeling or succumbing to. It is not a bad guy or bad object which, when once removed from the scene, will lead to our liberation.
I would just say, however, that PJ translates to the filmic medium something that Tolkien did brilliantly in the novel (nevermind the "forever" part). The end of LotR is supremely non-novelistic, what with the hero "disappearing" into an ambivalent, even ambiguous exile the nature of which is not adequately explained in the narrative. Sam's own ending, the ending of the whole story, is also an extraordinary rewriting of the novelistic convention. The utter domestication of the hero in that final paragraph, his being taken "back in" by the home and hearth is something that just does not happen in novels. In the 19th C, that scene would have been played out only in epilogue form and been presented as the achievement of the hero's journey, not the conclusion of his retreat from his journey. In the 20th C that scene would not be presented at all, except as a problematic and ambiguous "real life" moment to counter the supposedly "happy ever after" conclusion it appears to be.
What I mean to say is that Tolkien, in writing his book, gives us a conclusion that goes against novelistic convention. The drawn out series of endings (including the Scouring) leads to a rather anti-climactic moment... but only from a narrative/structural point of view, not an emotional one at all.
PJ does precisely the same thing but in filmic terms. Each of the "closing shots" that he gives in the serialised endings (Mount Doom, the coronation, the return to the Shire, Frodo's departure) is large, gorgeous, rounded out with large soundtracks -- they are typical Hollywood closing shots. The fact that they keep happening, I think, hammers home the idea that there is no one way for this film to really "end"; that the story the film is telling defies the easy conventionalities and sententious simplicity of Hollywood narrative. That it is all rounded off with a shot of the closed door of 3 Bagshot Row undercuts the drive to conclusion and understanding -- the final shot of the movies is not a narrative one in which things are explained in a final way, but a shot in which the ongoing story of Sam and Rosie is hidden from us -- they go inside to live their lives, the door closes, and we are left with the image of not being able to see what is going on.
I realize that this is not entirely on point with the original point of the thread, or with the current direction, but I wanted to put that up anyway. It does seem to me, however, that this careful drive to constitute the narrative as not finished, as escaping any final conclusion, works against the prologue's assertion that evil can be destroyed "forever". The vision of "forever" that we have at the end is one of continuing life and ongoing existence/change: no-one is so naive, I think, as to think that life is perfect. So while Sauron may be gone, we are still very much in a world like the one we live in: imperfect, ongoing, and in which bad things happen.
Going back to the topic, it is imperative to take heed on one of the quotes in The Silmarillion.
Evil will eventually grow again in the land. Nothing can be totally good and nothing can be so totally evil. As good prevails, evil will come to crush it, and as evil resides, good will eventually come to destroy it. It's an ever changing cycle of life.
