What really happens is the spells only have other spells to draw from and they begin to degenerate over time, eventually turning into chaotic eldritch horrors that randomly add limbs to people or adamantly refuse to discuss goblins or just shriek in gibbering madness. Our Evil Overlord sacrifices the dreams of children to keep the magic sustained and controlled, and soon the people can't even think or speak without the help of magic. And they think they're wizards even though they can't even read a grimoire.