Frequently we are not given reasons why having the villain's plan succeed would be a bad thing.
I will foster happiness throughout my empire so no-one will want to overthrow me. After all my interest is in power, not being a dick.
-- The Evil Overlord list.So there's the Big Bad, and he's a got a villainous scheme. It's big, it's ambitious, it'll make their name in villainous history. But for all we can tell, it wouldn't actually be such a bad thing. This guy might actually make quite a good job of ruling the world. It's often just left to be inferred otherwise. If the villain isn't even evil, more blatantly self-interested, this is especially evident. If all they stand to gain out of it is riches and/or fame, there's not much really to worry about for everyone else. Punch Clock Villain and Generic Doomsday Villain characters may be suspect to this, depending on their nature. General ways that this issue is avoided is that: