Actually some fans speculate that the Souls world goes back and forth between an Age of Fire and Dark. You can be the Dark Lord, but it doesn't mean the total end of the fire, instead the character reigns as the Dark Lord until another Hollow shows up and kills them and chooses to either take their place or reignite the First Flame. In truth, because Lord Gwyn bound the souls of humans to the bonfire and the First Flame, he has effectively created the ultimate contingency plan to make sure that even if his efforts failed with the first "Chosen Undead" there was always a chance a new "Chosen" would rise up and follow suit. Even Demon's Souls alludes to the fact that the conflict of the story itself is cyclical nature as not only has all the crap with the Old One happened before, but even putting the damn thing to sleep just means it will be just another few centuries before another asshole comes along and wakes it up to gain power. So far, every game Miyazaki has done has had a theme of cycles and the sheer despair it causes.

Dark Souls concept borrows heavily from Norse Mythology with the extinguish of the first flame being sort of like a less violent Ragnarok. Course in actual Norse Mythology, while Ragnarok meant the end of Odin and the Age of the Gods, it didn't mean the gods wouldn't still be around, as figures like Heimdall were prophesied to survive the great battle and become the new ruler of the gods in the next age. Similar in tone, while your Chosen Undead certainly takes down the major big wigs of the last age, there are still several gods in the mythos who seem to have simply disappeared. I mean what the hell actually happened to the real Gywnevere? She simply left with all the gods in the first game to someplace unknown and then DSIII has her partially pop up as the queen mother to the game's major location where she once again, does some stuff and skips town before all the crazy trout happens.

My point is that while I can get behind the original notion of DS being a one and done story, I feel the vagueness of the plot and the creativity of the writers and fans has more or less come up with good solutions to how the story can continue past the first game.