The thing is and I am not being contrary how do you know that humans are not dying out? Remeber this is a book that says two people (2^-3 of the noah survivors) made enought people in a handfull of generations to bring down a global apocalypse that killed everything on earth. Also remember there is nothing to say that noah was not the son or decendant of an incubus (or secubus if want to be fair).
I know it is temping to really use models based on today's observations. Noah was barely human. Now you can look at this 2 ways. One try to make a real nomal noah survive a local flood, or make a supernatural noah survive a global deluge. A normal noah can't survive the global flood he would be dead (from old age) before he finished the ark.
Also notice this. Everything on that ark sired different specise than the parents. That is more spectacular mojo than death by incest. It would classic monsterism straight out of X-men.
You clould pose a totally theological solution. The future generations was not closely genticaly related to noah and God inseminated all the females on the planet like he did marry, elizabeth, samsons mom, and those color changing sheep. He has shown a pinchant to get all up in that A*.
If this was a local flood it is obvious they met up with othere people and animals later on.