The narrator argues that his plan for the poor to raise and fatten their babies to be sold as food for the tables of the rich will have the positive impact of lowering both the abortion rate and the murder rate of children. Once the poor know they can make a profit on their babies, they will lose interest in aborting them. Likewise, they will be unlikely to commit infanticide on what the narrator calls "their bastard children" when they recognize that these babies will fetch money on the open market. Further, the narrator argues that the parents will take much better care of their babies once they realize what a valuable commodity they are.
Clearly, Swift is being ironic in having his narrator praise the possibility of lowered abortion and murder rates when, at the end of the year, these babies will be killed anyway as a food source.