I struggle with assigning infinite weight and unlikely possibilities. But we can stay in the every day world and ask what credence do we place on the time machine possibility, and why? All information I have read suggests that it is essentially impossible, and so effectively I don't worry about this contingency at all. And then, is there anything we could do to prevent the time machine scenario? I can't think of anything at all.
On the other hand, to focus on one particular risk, there are compelling arguments made by experts in the field of artificial intelligence (e.g. Stuart Russell) that say that we face real risks based on the current trajectory of the technology. Is there anything we can do to reduce the risks? That seems like an important question.
It is important to be sceptical, and whilst I don't place 100% credence on the likelihood of a truly catastrophic AI outcome, I do think it is worth taking this possibility very seriously. There is a strong chain of reasoning behind the claims, and they are worth engaging with, especially given the fact that we are in a position to influence the outcome.
> if you're going to be so abstract about it... what's wrong with a future with flourishing AI and without humanity?
We can park the abstract side for a moment and think concretely about such a future. I want humanity to flourish, particularly during my lifetime. Credible people are currently ringing alarm bells about the potential destructive power of a technology we are actively developing, and so I think on balance it is better (at a minimum) to listen and be vigilant than to dismiss it. Better yet, I think we should engage with the arguments and see if there is anything we can do to reduce the risk of a potential bad outcome.