In A Modest Proposal, why does the speaker brush aside his ideas for reform in favor of this horrible proposal?
Near the end of the pamphlet, the speaker lists "other expedients" that might help lessen the present distress in Ireland.
The reason that the speaker does not really want to consider possible ideas for reform is that Swift is writing a satirical essay, not an actual proposal for reform.
If Swift had the speaker truly consider the reasonable ideas, the force of the essay would be lost. Instead of being a satirical essay, it would just be a regular discussion of what to do about Ireland.
Also, if he considered reasonable alternatives, the whole "modest proposal" part would sound really dumb. You wouldn't say "well, on the Irish problem, we could sell their babies for food or we could raise taxes to help them." The two aren't really competing alternatives.
Finally, brushing off the reasonable proposals as he does actually draws attention to them. He's saying "we could raise taxes and help the poor, but that's dumb -- we should just kill and eat them." This makes the reader think "well, why not raise taxes..."
So, by brushing off reasonable proposals, Swift is making his essay a stronger satire.