What makes a successful campaign?
For me, a successful campaign is one that is able to tell a complete story. And to me this relates to one's awareness of time and circumstances. Everyone might be willing to be in the game at the start, but at some points schedules will change, people will have shifting familial requirements, the horrendous Manila traffic could make it impossible for some people to go.
I find that, locally, it is generally realistic to expect people to be able to commit for a year. If you try to create a story that spans longer than that in real life, there is a greater possibility of the game fizzling out before you can reach a satisfying conclusion. So typically I set the game to last for only a year.
Once I have the length and the frequency of the game sorted out, I have a good idea about what kinds of stories I can tell within the time frame that has been given to me. Here the system can help a. In D&D 5E, for example, I can expect a monthly game to get players to levels 7 - 8 in one year. With that information I can then plot out an appropriate big bad, figure out when the action will be at its peak, and leave "buffers" for things that players would want to do that is not necessarily tied to the main plot.
So far, this seems to work out for me. Games have a definite beginning to end times, and a complete story is told.