"Sailing to Byzantium" by William Butler Yeats is a poem, not an argumentative essay. Although argumentative essays by their nature have theses, most poems do not have arguments per se nor do they exist primarily to advance a single thesis. Instead, poems tend to have themes, clusters of associated images, emotions, and ideas that are central to the reader's experience.
The main themes of the poem are the relationships of the soul to the body and youth to age. The narrator begins by suggesting that much of the world is more fit for the young than the old; the young are caught up in romance and the life of the senses and pay little attention to the profound wisdom that Yeats associates with both age and agelessness.
The poem concludes by suggesting that while age involves physical decline, this should not be an excuse for a concomitant mental decline, but instead the soul can continue in the path of wisdom and creation, and participate in eternal beauty. The goal of humanity is to become an immortal creative spirit, casting off imperfect mortal flesh.