I actually like the WiSC. You've got to forget all the rational baggage of the real world, and do away entirely with the cumbersome concepts of evolution and plausibility.
It's a fantasy world. Magic exists. Demons and devils and fey and aberrations and gods and wizards exist.
Come on, tell me the evolutionary plausibility of a species of monsters like the demons? No two races in that species are remotely alike, as far as anatomy is concerned, except for a lot of anthropomorphic convergence that doesn't seem to be dictated by anything, really. Oh, and they can mutate from one form to another,
several times. And interbreed with anything. And they also evolved to be the embodiment of chaos
and evil.
Demons are really monsters of suck, in a rational worldview.
Now suspend your disbelief some more. Take a more magical worldview. Magic. You know, things like
spontaneous generation. Monsters are creatures that appear not because of "normal" events, but because there's an opening for them appearing. Just like a slab of meat left to rot will beget maggots, a dark and gloomy cave will beget shadow monsters. An abandoned necropolis will beget undead. A sinister forest near a town with naive and curious children will beget wolves-in-sheeps'-clothing. Any unexplored, unknown place will create monsters; because the unknown inspires fear, and fear is what creates monsters.