What is the primary consideration of the speaker's argument?
I would day that Jonathan Swift's primary consideration in "A Modest Proposal" is justice, specifically economic justice.
Near the end of his essay, Swift states his motivation:
[I have] no other motive than the public good of my country, by advancing our trade, providing for infants, relieving the poor, and giving some pleasure to the rich.
Of course, Swift presents his arguments with his tongue firmly planted in his cheek. He certainly doesn't really mean that little Irish children should be ground up and eaten. What he does mean is that the Irish people are suffering from poverty, and something ought to be done about it.