The aformentioned catastrophes are, perhaps, ways in which the human population would be greatly reduced, but not likely extincted.

An asteroid might do it, if it was large enough, or a super solar flare....Both AI and Nano are still too unknown to make a reliable risk assessment. Plague is unlikely, unless it is manufactured by the military --- that's just another unknown.

All out nuclear war would, indeed, wipe out billions; however, some specimens would survive --- think: deep underground bunkers stocked with a decade worth of supplies and the means to restart civilisation. The US government has such bunkers, so does Russia, so does China and so do a dozen other nations.....

Humans are hard to kill and they die hard....