Swift shows how greed corrupts the upper echelons of society in A Modest Proposal. While the wealthy hoard land like Monopoly money, the poor have no choice but to beg for tidbits in the street. Worse, it's not like those resources are being put to good use. Limitations on land development mean that peasants can't grow food to eat or sell.
Throughout A Modest Proposal, Swift draws an extended metaphor that connects consuming resources to (literally) consuming children. The wealthy have already eaten up the Irish land, so it's no surprise when they turn to the Irish children to feed their appetites.
Although the narrator in A Modest Proposal expects his readers to be motivated by greed, he is not greedy himself. Rather, he is genuinely interested in restoring Ireland to its former glory.
Swift suggests that greed is a universal human trait. Although the poor are not in a position to be greedy, he believes that they would gladly sacrifice their children to gain money.