One of the standards I suppose for a good ending is that the right people are doing the right things. I think Amity and Hunter have achieved this—they’ve each become a better person through these events.
But the question is, has Marco done the same? What specific benefits has this journey brought him? I don’t think the answer is simply that he gained the courage to be himself, nor that he improved his arrogant attitude, because neither of these were really issues, and I don’t see how they would impact his life on Earth.