The Borg want to add Earth's biological and technological distinctiveness to their own. Ultimate Luddite Revolution: Destroy all technology as to not be a tempting target for the technology. And since so many billions of humans without tech (food and health) cannot be sustained, a large part would die of starvation and disease so the Borg wouldnt bother to assimilate Earth's naked apes.
Did I just say continuing war is better than universal peace? Ok, now I'm depressed.