All it takes is one planetwide catastrophe to wipe us out. And Earth is overdue for one (it's about a 5,000 year cycle, give or take, go research it, it's kind of interesting).
This is why Stephen Hawking said that to ensure the survival of humanity, we MUST spread out into space. That it's far from a waste of money and manpower.
With all due respect but these apocalyptic scenarios are oftenly based on overly pessimistic projections from the past into the future. I know about the fragility of the earth's ecosystem and the dangers of super-volcanoes, virus pandemics and the possibilities of asteroid collisions but the first 2 scenarios aren't likely to be able to wipe out our complete species. Isolation will always let some individuals survive. The only real critical danger lies in the darkness of space and - in a few billion years - in the sun becoming a red giant.
It's not based on what if scenarios, it's based on the history of the Earth, stuff that has already happened. These extinction-level events have already happened, more than once, that's why they know it's about every 5,000 years, give or take. There have been extinction-level events long before humanity ever existed. If they didn't happen, then humanity wouldn't even be around probably, this would still be a world full of dinosaurs, or a world full of crustaceans and sea-life.
And Stephen Hawking is arguably the single smartest man on the planet, I'm not one that's about to question his reasoning.
So is that suggesting that our entire evolutionary process from tadpole to highly intelligent hairless up-right walking ape only took about 5,000 years?
The Ice Age is an extinction-level event (which we survived as a species). An extinction-level event doesn't necessarily wipe out everything on Earth, but does wipe out large sections of life.
However, there is a large possibility that an extinction-level event can wipe us out. All it takes is a large asteroid or other random celestial object, a shift in the Earth's climate, even a change in the atmosphere content (Earth used to be a carbon-dioxide heavy atmosphere in our distant past), and now we even add the human element with nuclear and chemical disasters. Something too big for us to outsmart.
Which is Hawking's point. It's not a matter of if, it's a matter of when the next one happens. When such an event happens, if we have spread out to the stars, then humanity is ensured survival. If we are all on the Earth during such an event, then we become just another footnote in the Earth's history.
