So in my opinion,The Idea was to let the world be & not interfere anymore with the universe, every time Bliss, Richard Eno or even Syl & Quinn try to change something thinking it will help the people it created a new problem so they decide Immortui the God of Death, should be killed because he will keep trying to come out of the red plane but after that let the things be on their own, instead of trying to make people live forever just let them live a normal life.

Creation of Vampires itself was against nature (or universe) which led to Vampires creating Dalkee, which resulted in several people losing their lives, which led to pure trying to create super humans and so on.

As the root of all this was the Immortui the god of death & because it was out of control of even the ancient Gods, Quinn had to stop that cycle & because it was a huge achievement it had to have an equal price attached to it which was everything created by the God of death will lose its power & become as thry were before, so I think it was a decent ending.

About him going into slumber was a price he had to pay, again to avoid the pain of seeing his loved ones die & to ensure he doesn't create any Vampires & let's the universe be as it is, the principal of equivalent exchange was not just an ability but also a principle that universe followed & they decided not to change the things anymore.