The Punchline to the Movie?

At the end, after we've seen Anakin go from slightly angsty to full-on evil for barely any reason-- he had a dream that Padme was hurt-- and has been convinced to murder children and become a Sith lord by Palpatine on the vague promise that only the Sith can defeat death, and that Anakin will need to look to the Sith for such knowledge as the Jedi can't provide it, Yoda casually mentions to Obi-Wan that oh, by the way, Qui-Gon Jinn had come up with a way to defeat death. Really? You couldn't have mentioned this earlier and basically prevented everything bad that happened in this film? It's like the punchline at the end of a skit the way Yoda drops that in there.