Volcanic eruptions, asteroids, nuclear weapons, etc. If you understand the world, no species survives here permanently. We will have our life cycle, but inevitably if we don't leave the planet, our time will come to an end due to any unknown catastrophe.
He's not predicting when, but rather making a logical statement based on the history of the world, where we are geologically, politically, socially, and what we are facing ahead.
We have around 200,000 years behind us, and maybe much longer ahead. But how we survive in the future is another question. Perhaps a natural disaster wipes out 90% of our species and we lose our progress, only surviving until the final hit happens.
Who knows? The point is, the only way to guarantee the survival of our species is to spread out in the Universe.
It really is not enough to just say we have time, when we really don't know how much...
It's one thing to disagree with his views, but you have to respect that he is far more informed on the subject, and his views are a culmination of all of his aquired knowledge.
Go ahead and refute it, but be prepared to give a very informed reason why.