I can see the civilization ending argument, although it seems like something resembling modern civilization could be rebuilt, maybe with less access to energy, but with less people you also need less energy, so I'm not sure how big of a problem that will be. Maybe without global industries there won't be the materials science necessary for space exploration.

I have a harder time believing independent actors can extinguish the human race. Biowarfare/terrorism is unlikely to do so for a variety of reasons, and nuclear warfare only has the potential to do in superpower level conflict, and even then I don't think it's certain.