About the demons thing:
* I always feel like Quelaag's web-house-thing blocks the entrance to the demon ruins and disallows them from leaving.
Let's build a little scenario here: Maybe when the chosen undead (i am disappoint in my spellchecker for not knowing the word undead) stays away for a couple more centuries, the fair lady will die from her blight-pus induced illness and Quelaag might leave her domain, since she doesn't need to take care of her sister anymore. That leaves the web-thing uninhabited and subject to decay. Over time, the passage might open up and allow the demons to expand into blighttown. From there, the taurus demons and chubby demons (you don't see those in the demon ruins (other than the firesage) but they have to come from somewhere) will be too big to continue, but the capra demons might climb the wooden platforms and escape into the rest of Lordran!
The other answers mostly overlap with Doughguy, but i'll list them anyways:
* It's debatable whether Gwyn can burn out. When a human dies of old age, is that his soul burning out? If so, Gwyn's soul (since he doesn't have a lord soul anymore) could burn out too. And since the witch of Izalith isn't around anymore to make a third one, that will end the age of fire. But as i said, debatable.
* I don't think there's anything else Gwyndolin can do. If he could, he wouldn't be hiding behind an illusion of his sister
He'll prolly just continue doing what he does until all his followers are gone. Which once the age of darkness comes, is only a matter of time.
* I think people have been debating whether Pinwheel stealing Nito's power was a one time act or if he continues to do so until the chosen undead kills him. I'm not sure if they ever came to a conclusion. If it's the second option, then a couple more ages might give him enough power to become worthy of the title boss
* From what i've seen, Seaths experiments are more an act of madness than an act of science lately. It's unlikely that he'll discover anything. He had almost a millenium (who knows when exactly he went crazy) of sanity to find a way to create the scales of immortality and didn't succeed. Best he can do right now is create more messed up creatures.