Actually, doesn't the poem begin with the rhetorical question as well? Still, isn't it striking how many of Yeats's best poems end with a question? What, you want an example? Of course you do. How about "The Second Coming," which ends with the question, "And what rough best, its hour come round at last / Slouches toward Bethlehem to be born?" Or "Leda and the Swan," which also ends with a question. Yep, it's a definite pattern.