I've been playing and running D&D since 1994; the number of campaign arcs that I've successfully completed is immense. Full campaigns? A much smaller proportion, depending on how you count them.
For example, my weekly game just hit an awesome climax (complete with nail-biting tension featuring a character being savaged by spells at close range while repeatedly missing on attack rolls, only to land a natural 20 just at the crucial moment). If we just ended there, everyone would call that campaign a success, but why stop there? The newly defeated villain has friends' allies, and accomplices, and the players still have their own goals to fulfill.
So is that a campaign completed successfully, a campaign still ongoing, or a campaign that's unsuccessful?
In other games, such as HeroQuest, the "completion" rate has been much higher; we've worked through like 4 or 5 of the Quest Books and are still heading through another. Of course, those are the same characters all along, so maybe that's another "ongoing" or "not completed" example.