Consider the following passage: i can think of no one objection, that will possibly be raised against this proposal, unless it should be urged, that the number of people will be thereby much lessened in the kingdom. what is the obvious objection that swift ironically omits in this passage?

The obvious objection that Swift ironically and consciously omits is the inhumanity of eating newborn babies or selling them to wealthy aristocrats to be cooked and served on their dinner tables. The fact that Swift proposes such a thing, explicitly saying that no one will object, is the ironic provocation at the core of "A Modest Proposal".