"Well, whether that's true or not, they're going to shatter the planet if they keep fighting. We gotta stop them or else, right?" "They're trying to lure us over, though, aren't they? If we don't engage her, Feldway's plan is gonna fizzle out. Isn't that the best move to make?" As Guy explained, this planet was created through the power of Veldanava and therefore wasn't going to be catastrophically torn apart or anything. But Milim's power was still rising, and if left unchecked, she'd encase the whole planet in a thick layer of contaminating magicules. ...Well, if there's still gonna be a planet at the end of the day, then great. And I guess it made sense. After all, even Carrera's magic wasn't the sort you should really use on the surface of a planet. We only got away with it because we were on a world that could withstand such lethal force. Otherwise, at best, some attack from her would've bent the axis of the planet by then.