When is a storyline complete?
Okay, the conflict has been resolved. Big deal: life is conflict. Like streetcars, another conflict will be along any minute.
So the Bad Guy has gotten his comeuppance. The world has been saved. Yawn.
Prevent one nasty End of the World, and the universe will cook up another. The universe is the ultimate Bad Guy that way.
If all that wasn't "The end"-defying enough ... Wait! There's more!
There's that support character with a fascinating personality or a unique point of view. What's her story? What if she should take on a more active role?
Then there are folks like you, challenging the author, "But what about ...? Just suppose ...? Did you ever consider that ...?"
Hence: sequels. Prequels. Companion novels. Companion series. Reboots. Re-tellings.
If flying fingers entering "The end" won't conclude a storyline, what does? Ennui. The cascade ends when the author, or the market, has had enough.
And if the author or the market loses interest in a storyline before you, the reader does? That is a real bummer.
(The cover thumbnails here are from SF novels that became popular series. To be shown does not imply any opinion about the optimal length of these series.)
Which are your favorite fictional series, SFnal or otherwise? Which series or universe met an untimely end, or continued beyond its expiration date?