imagine Earth was destroyed and only 6 people survived plus a dog, plus one person from an alternate universe (which is a nice excuse since, well, it's a freaking alternate universe, different things happen)
out of the 6 billion (close to 7), plus the countless species of animals and plants and etc... only 6 people and a dog survived. do you think that would be enough to make the destruction of Earth not that big of a deal?
PS: not to mention that out of those people, there's only two females and they share similar if not identical DNA and are related to one of the males, which accounts for a not very diverse genepool and a cultural background that probably doesn't even account for 0,00001% of all of that planet's history and culture
@jorgevy: Never said it wasn't a big deal, just not as big as one at the start when there were only 2 survivors from it. Now were up to 8, and who is to say later that more won't show up?
It was kind of the same before the New 52.... Minus H'El and depending on the story Zod's lackeys. There's also Kandor. I doubt anymore will be introduced.