How would you know that human population will go down to zero rather than stabilise at a lower point or start to grow back after a period of decline? All projec

How would you know that human population will go down to zero rather than stabilise at a lower point or start to grow back after a period of decline? All projections into the future are, of course, just hypotheses but "people will have children as the vast majority of them are biologically hardwired to do" strikes me as a lot more likely than all of humanity just deciding not to keep having children.