‘For ever more’ in line 38 now refers to emptiness.
It is as if the vivid, fresh mood of stanza three has been reversed.
By being preserved from the passage of time, the characters on the urn are also trapped by it, never being able to reach for new joys in the future.
Preservation from time forbids growth which is a key element to life itself.
Even the understated sense of inevitable loss in the final line does no seem tragic as the birds will return as the seasonal cycle continues.
Instead of joy always leading to sorrow, sorrow will now lead to joy.
Turning to Poison while the bee-mouth sips: Beauty must die, joy is fleeting and the flower of pleasure will turn to poison.
This seems to echo the sadness found in Grecian Urn.
In the third stanza, he looks at the trees surrounding the lovers, and feels happy that they will never shed their leaves; he is happy for the piper because his songs will be "for ever new," and happy that the love of the boy and the girl will last forever, unlike mortal love, which slowly turns into "breathing human passion," and eventually vanishes, leaving behind only a "burning forehead, and a parching tongue." In the fourth stanza, the speaker examines another picture on the urn, this one of a group of villagers leading a heifer to be sacrificed.
He wonders where they are going "To what green altar, O mysterious priest...", and where they have come from.