On a personal level, I consider it bad storytelling to set up a plot that you have no intent to resolve. Until we get more info about how the movie is going to play out (I am still thinking rehash of the story), that will always be open-ended. It just bugs me because I see it in other anime to the point where it feels cliche to me.
Now, that being said, maybe that ending fit the series. Alto's first love was flying, especially in a real sky. Maybe what won out in the end was completing the dream he had in acheiving that goal. All in all, it wasn't such a bad way to end Frontier, but I just wish it hadn't used the cliche ending that has been used in other anime.