1. Kinzie says the Zin have already abducted the entire population of every major city. That's a whole lot of people.
2. During the mech sequences, they make multiple mentions of how there are a whole lot of pods everywhere.
3. Call Kinzie and Shaundi as homies at the same time, and Kinzie will explicitly state that the Zin captured enough people to repopulate the human race.
Yeah, trust me, they've hinted enough to where you know the human population will never truly die, just the Earth itself remains the issue. Will Saints Row V go through time to save the planet? Or will Volition go fully Sci-fi and have the Human Race repopulate on another planet? Only Volition will know.