I think it should have been revealed that he never could actually go back in time, and just had to settle for defeating Aku in the future and living out life that way. Would have made the ending less dumb and uh, you know what not happen which is why it was dumb. I mean, the only real reason we expected him to go back to the past is because of the song. Why does being transported to the future equate to being able to transport back? For all he knew, he was just in suspended animation for however long.
Still, despite the lack-luster ending, I do like that it had an ending.