It certainly wouldn't be the best thing that ever happened if you were one of the 70 percent that got wiped out. It might very well be a beneficial happening from the perspective of those who survive. A good analogy is the slave trade. A black slave from Africa, taken into captivity and killed horribly on the unthinkably cruel and brutal voyage to the Americas certainly wouldn't have thought the slave trade a good thing. His American ancestors, on the other hand, might perceive things differently. While they still couldn't call the slave trade "good," they could at least acknowledge that its existence benefited them.
Interesting analogy. Gosh, I wonder how many Blacks in the USA thank their lucky stars every day that their great grandfathers were slaves. My guess would be, not many.