How is the theme of decay expressed in the poem "Sailing to Byzantium"?

In the poem "Sailing to Byzantium," decay is expressed through the mortality of humans. The speaker ponders the decaying and aging of human flesh compared to the ways in which one can figuratively achieve immortality through artistic expression. A city such as Byzantium, in which the arts and pursuits of intellectuals flourish, immortalizes the creative and intellectual expressions of mortal humans. The speaker notices how those who are still young and able to fully enjoy the sweet, simple joys of life and love do not tend to consider decay and death. Meanwhile, those who are old and have aging bodies must grapple with their own mortality.
The speaker grapples with the reality of mortality and decay through considering the ways in which immortality is achieved through artistic and intellectual contributions to humanity. As such, the speaker seeks to travel to Byzantium to pursue this figurative immortality.