Role-playing campaigns can learn much from the failures of television series. There is nothing worse than a long-term campaign that drags on past its sell-by date, slowly rotting like old milk. Plots grow convoluted, game master’s creative well runs dry and epic power is in the character’s hands. Rather than squeeze the last drop of fun out of a campaign, retire it. Go out with a bang, blow up the main city, end the world or otherwise put a period on the sentence that was the campaign.
Contrast this with the “squeeze every last dollar out of the franchise approach” too often seen in genre television. I doubt you will find anyone that mourned the “X-files” after season five. It was not a terrible show, it just lost its fizz. Eventually fading away in a cloud of sub-standard acting and tedious “monster of the week” episodes.
If you can run a long campaign successfully, then do so with my blessing. That said, twelve memorable game sessions are a better legacy to leave your players than 50 forgettable game nights.
Trask, The Last Tyromancer