While yes it is the return of a king with the blood of Numenor flowing through him, but that king is a man. Like you said, the elves are leaving. The future belongs to men to make for themselves. In many ways though, it isn't really a positive or negative ending. One of the themes that runs through Tolkien's work are that time passes, and things change, and nothing ever seems as good as it was but in turn those times never seemed as good as things that came before them. It's the story of the human condition. But there is no going back.