The simple answer is that the Hydaelyn/Zodiark storyline is concluded, and neither will be making future reappearances for the sake of making a fresh start. They reserve the right to Emet/Hyth cameos, as well as introducing assorted sundered Ascians as new storyline hooks.

I know that this 'past life' business creates some philosophical questions about the nature of death, but you are still you even if your soul happens to have a few residual memories from a previous user that weren't completely erased during the factory reset. Hermes and Amon were still different people, even if Kairos unintentionally burnt Hermes' memories of Elpis on their soul, and even that's now been addressed. You're still dead if you're dead, irrespective of how your soul gets utilized after the fact.