Out of all the timelines there are, this may indeed by the darkest timeline. For a show that was set up to be optimistic to have it end in such a grim way is a gut punch, but in a good way.

And I suppose it also answers a question the show couldn't ask: how long can humanity run away from extinction? Even if the human population of 12,000 AD found a new home in the far future, how long until the Earth becomes uninhabitable again and the whole process starts all over again. In the end humanity, like all creatures, will meet its end to extinction.

Also, are the giant crabs an homage to The Time Machine. I believe in the book they inhabit the last time period the Time Traveler visits after fleeing the Morlocks near the end. Nice little Easter egg.