My explanation for a lot of Athas’ problems gets into some VERY non canon territory but I figured I’d throw in my two cents regarding the Gray’s origins.
I subscribe by the notion that Athas used to have gods, but that they came in the dawn years of the green age and that the other races aren’t descended from haflings but brought by their respective gods or migrated to Athas through planar or spelljamming means. To make a long story short the gods ended up dying or abandoning Athas. The sun didn’t end up the way it did due the pristine tower drawing THAT much of the sun’s essence, but because the chief of the Athasian pantheon, coincidentally the sun god, died. This is the same reason the sea of silt is the way it is, because the god of the seas perished. The gray exists because there is no god of death ferrying souls to the afterlife. Normal arcane magic doesn’t exist in Athas because there is no deity preserving the balance of magic. This cataclysm, which I called the Iconoclasm is the reason Athas is cut off from other planes, the death of so many deities in so little time caused a lot of unexpected consequences, some obvious, some no one could have predicted.
I’m not intending to take away too much of Athas’ problems from the Cleansing Wars, just offering an explanation for some of the feature of Athas that make no sense to attribute to defiling. Instead it hints at a greater history and that the genocides of the sorcerer kings are but one piece of a greater puzzle.