I sometimes wonder what would have happened if series such as Space Quest and Firefly wouldn't have come to such an abrupt end. While Firefly was given a movie sequel and its own roleplaying game, and there have been a few full-length fan-made Space Quest games, it's just not the same as having Firefly: Season Two or an official Space Quest VII. Yet, sometimes I'm glad certain series stopped where they did, even if the conclusion was less-than-satisfying.
If a particular series has enough untapped potential to keep it going--whether that means a new Stargate spinoff series or another Dune novel--I'm generally happy to see it continue to thrive. Even with my absolute favorite fandoms, however, I acknowledge that there usually comes a point where it's time to either quit while you're ahead, quit while you're way past the point where you should have quit, or take the series in a totally different direction. I'm normally fine with any of these three options... as long as the series gets a fitting and satisfying conclusion.
I find that closure is very important to me, both in my fandoms and in the rest of my life. When I say goodbye to someone, I don't like to leave anything negative hanging around in case I never get to seem them again to make things right. When I write a review or express complex opinions in my posts, I like to be as thorough as possible so that, long after I'm gone, my thoughts on a matter will be completely clear. When I run a D&D campaign, I like to make sure any and all loose ends are tied up by the end of the campaign, unless I'm intending to make use of them in a future campaign.
This is why the 2009 Star Trek reboot bothered me so much; I didn't feel that the classic Star Trek universe had received proper closure, nor did I feel that the universe had been drained of ideas to the point where a reboot was necessary. Really, the movies and TV series only covered a few years of early Starfleet history, a few years of the journeys of Kirk's Enterprise, and something like two decades of everything that happened with TNG, DS9, and Voyager, which all overlapped to some degree.
Where was the series about the cadets at Starfleet Academy in the decades between TOS and TNG? Where was the movie starring Klingons as the protagonists? Novels, comics, and video games continue to fill in the gaps, especially with Star Trek Online, which confidently marches on after the events of Star Trek: Nemesis; clearly, Star Trek wasn't so desperate that it needed a reboot. There's a sequel to the new movie in the works, and because I'm not gung-ho about continuing this alternate universe, I'm starting to feel left behind as a Star Trek fan.
Same deal with Dungeons & Dragons; I used to check in on the official D&D website once a week to look at all the new sourcebooks and miniatures previews. Now that 4th Edition has taken over, there's not much point for a 3.5 fan such as myself to bother. I can't use any of the new books, and even the miniatures are becoming less recognizable because they're drawing their inspiration from sourcebooks I've never seen.
The problem inherent in any long-lived fandom is that stagnation can kill the fandom, and change has the potential to kill the fans' interest in the fandom. Perhaps it's better to have a good, short run; that's one of the many reasons I tend to prefer 13-episode anime series over other shows that have been running since before TV was invented. If your plan from the very beginning is to have a limited run, I suspect you're more likely to accomplish everything you want to accomplish with your series, with less fluff and more focus.
Maybe we'll never get to see Sam Beckett return home. Perhaps the final Final Fantasy is just a few sequels away. It could be that our great-great-grandchildren will pick up a copy of Watchmen and say, "This is where the series started!?"
As we learned from Indiana Jones, Chinese Democracy, and a slew of others, there's no telling when something we love will come back, for better or for worse. The Simpsons is proof that certain fandoms can continually disappoint some of their fans and still keep going. For all we know, we might never see a sequel to The Dark Knight.
Maybe it's all for the best. The Dark Knight Returns Without Heath Ledger could forever tarnish the reputations of everyone involved. Indiana Jones 5 could end up being everyone's all-time favorite Indiana Jones flick. There's no telling where a fandom might go from here!
...Which is exactly why I've jumped ship on modern-day Star Trek and D&D.