We really can't go back...
The stubborn old merman refused to believe the retellings of the other two tribe leaders, insisting that Leon take him down to have a look. But after personally passing through the sea eye and seeing the world on the other side, the old merman's hopeful thinking was ultimately crushed mercilessly by the cold reality.
Although the sea eye they came through hadn't disappeared, the other end was no longer the seafloor of the present world, but an endless cold sea in the Cold Spring Realm. Even trying to shuttle back from the Cold Spring Realm through the sea eye couldn't reach the present world or the Tidal Realm and would still end up returning to this cold new plane.
It's over, it's really over this time.
After being shivering hauled up out of the sea by Leon, and seeing the same despair in the eyes of his two old friends, the old merman couldn't help but close his eyes in pain and collapse weakly onto the ice pillar.
