Those who think human extinction is impossible are incredibly, fantastically naive.
Just as those who think AI and nanotechnology aren’t real threats are also complete naifs.
Their argument to shift scales a bit amounts to declaring, “Well, I haven’t died yet, therefore I will live forever!”
Anyone can see how ridiculous an argument this is. Yet when you aver that human activities are quite likely to lead to human extinction (especially while we are limited to one interconnected planet) on a medium to long time frame, well, then you are just nuts.