Yeah, all the GOOD harbinger of the apocalypse mention a way to REACT to the apocalypse in some constructive way. Repent from your evil ways, send a team of oil rig workers to nuke an asteroid, something like that. Any second we could be wiped out by an unpredicted supernova somewhere close by, or a huge-ass solar flare, or a really big volcanic explosion. And not only can we DO NOTHING about it, we don't even properly understand the mechanics of what would make it happen!
And if 80% of all life on earth got wiped out, the remaining 20% would probably be the things that live on the ocean floor next to a volcanic vent, can hibernate for six thousand years under a glacier, live inside bedrock subsisting on sulfur deposits, or in some other way cannot be killed. Not humans. While living in a giant bunker with an infinite energy source and your own ecosystem makes you pretty much independent of the rest of the planet, you won't have anywhere to run when some act of plate tectonics smashes your house like an empty eggshell. And space is just as bad, with deadly projectiles and absolutely no usable resources. Well, both of those could get you across a nuclear winter if you had enough redundancy, but they need preparation time.