Almost no one believed Y2K would kill everyone, or even a significant (>50%) percentage of the population. Beyond a few people talking about a Nostradamus prophecy or some such, people maybe were concerned that airplanes would fall out of the sky and elevators would plummet.

Can you link me to any written evidence of someone saying one of these would kill everyone? or are you speaking hyperbolically?

“Climate change” is a bit squishy since yes obviously a certain intensity of climate catastrophe can kill everyone, but no scientific prediction has said this is likely to be the case.