For example, there might be a Good/Evil zero sum rule. So every demon that enters the world allows an angel to enter to balance him. Which btw explains why the forces of good are so reluctant to intervene directly, they don't want to allow a corosponding number of demons to intervene elsewhere.
Who's to say that they haven't? In my settings, it tends to be more subtle than rampaging demons eating people all over the place, though. My pantheon is largely made up entirely of demon lords. They mostly keep the rank and file from interfering with their mortal fields, because that means a poor crop of new souls to refresh the ranks, after all.Think about it: in the D&D universe--whether the old Great Wheel set-up or the new cosmology--you have countless deities of every possible alignment and ideology; you have planes of purist evil, including the near endless Abyss that is teeming with demons, not to mention the many epic level adventurers that seek their own immortality.
How does the world stay in balance? A specific question might be, why hasn't the maw of the Abyss opened up and flooded the world (whether your homebrew, Eberron, Greyhawk, etc)?
Is there a canonical reason? What about yours, if you've thought of it?
Grimdark? Where is that on the continuum between Shadowfell and Scarytown?But yeah, as malraux said, the PoL setting is pretty grimdark.