That is indeed a good way of hedging your bets, and I agree that it works very well in the context of the world ending. However, I'm not sure Harry can even call it "very likely" that the Death Eaters won't die within the day; the probability that someone ends up killing them is nontrivial. (Of course, I'm aware that your suggestion doesn't include anything about the Death Eaters' well-being; I'm just talking about the statement I originally suggested, more for the sake of argument than anything else.)