In "A Modest Proposal," why does Swift separate himself from the narrator at the end, and how would his readers view it differently if he had not done so?
Swift does not exactly separate himself from the narrator, at least not in the sense that he "breaks character." At the end of the essay, Swift has his narrator reassure the reader that he has no selfish motives in offering up such a proposal, not having any children of his own young enough to sell for food. But in the penultimate paragraph, Swift offers a devastating account of conditions facing the poor in Ireland:
I desire those politicians who dislike my overture...that they will first ask the parents of these mortals, whether they would not at this day think it a great happiness to have been sold for food at a year old, in the manner I prescribe, and thereby have avoided such a perpetual scene of misfortunes, as they have since gone through, by the oppression of landlords, the impossibility of paying rent without money or trade, the want of common sustenance, with neither house nor cloaths to cover them from the inclemencies of the weather, and the most inevitable prospect of intailing the like, or greater miseries, upon their breed for ever.
"A Modest Proposal" functions as a satire on a number of levels, and it is often rightly appreciated as a critique of those who sought rational, "scientific" solutions to human problems without weighing moral concerns and the human costs of these solutions. But this devastatingly blunt statement makes it clear that Swift wants his readers to know that the Irish poor, largely through the actions (and inaction) of their English rulers, are subjected to such wretched living conditions that death might be a preferable fate. This passage trains the readers' attention on their plight, which might otherwise have been overlooked by the shocking nature of the satire. So while he does not exactly abandon the narrator's voice, he makes it clear at the end of the essay that the sad condition of the Irish poor remains at the forefront.
Swift separates himself from the narrator to emphasize the irony inherent in the satire. Swift creates a fictional narrator who appears to be a hardened, ruthless economic expert intent on using kids to supplement the diet of the starving Irish population. However, at the end, Swift abandons this narrator because he no longer wants to use the 'eat poor kids' rationale. By now he should have effectively shocked and angered his audience into listening to him. At the end, he returns to topics that he has formerly brought up to the leaders of the country (such as affordable housing, using Ireland's own natural resources instead of importing from England, teaching their farmers how to farm effectively, getting rid of the absentee landlords, and so on). All of those are rational solutions to Ireland's plight. However, no one listened to those. So he concocted this cruel and efficient narrator to propose an extreme solution. By reminding his readers that he is not really supporting eating innocent children, he hopes to make them take his rational solutions more seriously.