Well, i can tell that you're a 'glass half empty' person. Yes, the Zerg never stop coming. Just as entropy will eventually end the universe all on it's own. By living in space, humanity will ascend to the next stage in it's evolution, when our souls are no longer bound by gravity. And naturally, this can't be done on Earth. You don't wanna fight the Zerg? Fine, we'll hook you up with some cryo tubes and a sleepr ship, have fun traveling to Andromeda. But it is humanities destiny to leave Earth.