Abyssea did something clever with Prishe, and I didn't even realize it until just now
To establish a baseline, Abyssea asked the question: "what if the worst-case scenario happened in CoP?" The heroes of Abyssea lost, Selh'teus was absorbed by the Keeper which allowed its transformation into the god Shinryu, and Prishe was forced to live an immortal life rather than regaining her mortality.
It's the B storyline to the A storyline we played out. But aside from just switching the best-case outcome to the worst-case, it also switched Prishe's role in the story. She went from an immortal wishing for an "end," to an immortal who now watches over Abyssea for all eternity, alone in another dimension as she's left grieving for what she's lost.
Thinking about that, her role has now become analogous for Altana, rather than Promathia's incarnate, which I thought was fascinating. I hated how Abyssea concluded Prishe's storyline for the longest time. But once I had this realization, I've come to appreciate it as a deeply tragic, thematic mirror of Altana's situation.