It should be noted, and because some people still believe baseline humans and mutants are separate and not the same species, humans can still give birth to mutants with either their X-gene active or not and vice versa. So really the threat makes no sense because the only way to kill off mutants is to kill off humans, it was the premise of Days of Future Past was it not? That the Sentinels figured that out and why they took over the world if memory serves.