I think the main reason the "True Ending" is jarring to us is that the future humans have quite different values from early-21st-century humans -- they rape each other and intentionally put their children through painful experiences (maybe they took a wrong turn somewhere along the line, as one of the characters suggests). Our own values are probably closer to the Superhappies than the Future Humans, so naturally we prefer the happy ending.
they rape each other and intentionally put their children through painful experiences
This is just plain wrong on both counts. The non-consensual initiation of sex becoming equated to non-consensual initiation of conversation is very reasonably explained and investigated elsewhere on LW. (I still suspect that it would work a little poorly, and could be a naive policy.) Humans are probably more careful then now about letting their children bump their knees in all sorts of ways. I imagine their upbringing to be very awesome indeed, more fun and less painful then today, and totally worth it.
I have just posted an essay analyzing "Three Worlds Collide" on TV Tropes.
Comments welcome.