Demons respawn but the Demon King is an actual toddler, most likely a reference to how the archetypal DQ Hero would respawn back at the human King if he fell in battle.
It's not like the demons are guiltless in their methods, particularly because they do have some leeway with their lives and...
Miku doesn't actually care about any of that. All Miku cares about is Miku, so she made a world where she could be happy - that is, live in luxury and without fear of the Idaten.
She's just being edgy and pointing out that even if things are peaceful now, ultimately nothing lasts. And she's...