The only ways I can imagine the end of the world not being a downer ending are:
1) There was an even worse alternative in the cards (since this is LessWrong, let's say it was 3^^^3 years of agonizing torture for the whole human race) and we managed to dodge it by dying. But the fact that we were even faced with such a choice sounds pretty down-ish already.
2) You're some kind of negative utilitarian. Or perhaps a Buddhist.
3) From a certain point of view, you could describe the emergence of a new utopia as "the destruction of the old world" or something like that. That's how they got away with it in Alexander Senki. Or else "the end of the world" refers to something that would have happened anyways, like the end of Sol or the heat death of the universe. Thus, it will happen if Lily is nice to Petunia - it just neglects to mention that it would also happen if she wasn't nice to her sister, too, thus letting the listener believe there is a causal connection where none exists.
Promethea used a version of #3, saying the end meant the end of our ideas about the world.
Thinking about this, it occurs to me that perhaps the centaurs saw history along this path proceeding up to a certain point in time and then (it seemed to them) stopping. They might have explicitly told Lily that they couldn't perceive anything after the Event Horizon, and the subtlety got lost along the way to the second repetition.
Update: Discussion has moved on to a new thread.
The load more comments links are getting annoying (at least if you're not logged in), so it's time for a new Harry Potter and the Methods of Rationality discussion thread. We're also approaching the traditional 500-comment mark, but I think that hidden comments provide more appropriate joints to carve these threads at. So as of chapter 67, this is the place to share your thoughts about Eliezer Yudkowsky's Harry Potter fanfic.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six. The fanfiction.net author page is the central author-controlled HPMOR clearinghouse with links to the RSS feed, pdf version, TV Tropes pages, fan art, and more, and AdeleneDawner has kept an archive of Author's Notes.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: