Think it's been mentioned before, but the world is not the people but rather the physical planet itself. Those that seek to destroy the world like the demon lords are a threat to it, meanwhile those that seek only tyranny and a playground for their immense powers aren't actually all too much of a problem. They can always repopulate and are just like any other animal that lives or dies. The world doesn't truly care about them
I don't particularly disagree, but why does a demon lord automatically want to destroy the world? It would make sense in stories where the demons are creatures from another dimension/world, where they wander from one world to another, consuming everything in their way, leaving behind only utterly dead worlds when they move on to the next target. But if it's a demon lord of a particular world, sticking to it like any other denizen of that world, it seems to me destroying the world is not necessarily the goal, even if destroying civilisations was. But like you said, civilisations aren't the world.
Demons, and equivalent, in many real world religions and folklore aren't most often after the destruction of the world, I believe. They just pester humans, corrupt them, and do that sort of things. Though there would be exceptions, of course.
