How is the assumption that "ending of human life would not be so easy" any better than the opposite? If they're equally valid, then its rather fair to take the opposite view because it is more cautious.
We have 4 billion years of evidence that nearly ending life on Earth is easy and has happened multiple times. You would not be standing here today if that were not the case, the previous die off put the dinosaurs to the side and made space for mammals to become what they are.