Why do you think that it's so certain? There are moments in history were the number of humans in existence numbered in the thousands. What kind of event are you imagining of that is certain to occur that will kill every single last human?
i don't think the primate tree is going to be terminated that soon, but i think it might be pruned.
And I think our version of seafaring, spacefaring, will be fruitful over a timescale of thousands of years, and maybe much shorter than that.
Our ancestors walked from Africa to the corners of the globe. Today, the cheapest and safest way to travel thousands of miles (as well as the most common) is to hurtle across the sky at nearly the speed of sound at altitudes that would kill you if not in a pressurized volume. The energy used for this, a round trip from one side of the globe to the other, is about the same as needed to place you in orbit (with hyper-optimized chemical rockets). I don’t see why we think the last ~50 years is the end of such progress of our technology.