First, define “successful”. Also “campaign”.
Back when we played the post-college Shadowrun game, I had the whole campaign outlined ((Remember, this was back when I was still thinking of roleplaying games as primarily the GM’s story where the players enter with their characters to see what happens, rather than the collaborative play style I enjoy now.)) to create story arcs, season arcs like a television show. Each season started and ended on certain points. When we played that game, we went through the first season and had a good ending. Then we tried to do play-by-email once we started all moving away in the second season and that one didn’t have a satisfying ending.
My answer? Endings make a successful campaign.
Plan for an end point.