So it sounds like you're saying once a certain species eats a certain amount to become the dominant species they just go dormant or die out, but wouldn't they eventually just submit to their sense of hungry and eventually just go and eat the weaker species to avoid them becoming the dominant species?
What makes these beasts different from the intelligent species like humans, dwarves, elves and angelics? From which I mean, why don't they need to devour stuff to evolve. Why weren't the gods magic crazy on them? Why are they normal? Why can't one of those beasts eventually devour so much, and evolve so much that they eventually become "normal" as we so put it and they just need to eat to sustain themselves?
For starters, eating a weaker species wouldn't just automatically make them weaker, though it would be much less nourishing. They just wouldn't be able to fuel themselves with enough energy to stay active. Thus they can either go into dormancy in order to make the energy they do have last longer in an attempt to buy time, or simply keep on going until they inevitably die.
And some of them DO become highly intelligent, it's just they very seldom take on a humanoid form.
Demonics are part of the food chain, being wild spirited shape shifters they are almost a perfect fit for the world. They thrive on chaos and all.
The Angelics are part of that extreme amount of magic in the air, and thus a perfect fit on their own way. It's just their abilities come with a steeper price.
The Draconics are similar to the Demonics in that they fit in quite well, just not as well. Their love of violence means they see the loop of devouring others as somewhat of a challenge.
The Shadoxen are unknowns.
Humans and such of course are out of place and only have the advantage that they seem to have a relative cloak of anonymity around them.
The reason why they do not "evolve" or change a great deal from devouring other beings, or at least the main reason, is that they were made or at least altered to be that way. They are almost direct products of the mad gods. Although a powerful enough Demonic can copy more than just physical form.
I've thought about what you said about blood and all by the way, and in the wake of Bloodborne I figured it wouldn't be too far fetched to have this be one of the Angelics research and experimental projects.