I think it's worth noting, that the Replica walks away at the end at the end of Sora's story mode. Namine is right there, and she very well might need him in the future, but...he walks away. To me that sends a very clear message. It says he's not content with the way things.I see what you're saying but I think they needed to say "Now that I've protected Namine, I've got nothing." Riku goes to lengths prior to this to say "Even though my memories are false, I'll cling to them and that will make me happy," and I don't think they can undo that many blunt statements of intent with mostly off-screen inference. And even that's just one way. More cutscenes in R/R, like actually showing Axel tempt Repliku, would have been even better!
Although... now that I'm thinking about it, my impression of Riku might be corrupted by the remake. Riku saying that he was fine was a remake addition, wasn't it? Hrm, so it maybe the remake that's got me thinking Repliku was in better shape than he really was? Darn it, I liked that edit!