I was rereading some parts of TLO and noticed that the curse Hades out on Delphi is much more interesting that I thought:
"I swear," Hades said, "as long as my children remain outcasts, as long as I labor under the curse of your Great Prophecy, the Oracle of Delphi will never have another mortal host.
Hades said, "As long as my children remain outcasts," the only motive why Rachel could receive the Oracle is because Nico wasn't an outcast anymore. But what if he becomes one again? The Oracle would be stuck again in Rachel's body?
We could pull the part in which Hades talk about 'until he isn't stuck because of the Great Prophecy anymore', but he is very literal with his words in most of the book, so I can't see that he only mean his children don't being outcasts for a week after the prophecy.
This means that if the next generation started to treat Nico as an outcast, the Oracle would be stuck again. And since the only ones who know about this curse (in it's integrity) are Percy (who seemed to forgot or pass as resolved), Nico and Hades, I don't see they resolving this any time soon.
Imagine the pain, the confusion, the moment they notice that the Oracle is stuck again; the moment in which Rachel feels like her antecessor; the moment they try anything to understand what happened.
They can get angry at Hades, but he didn't curse the Oracle this time. Is the same curse, playing again because of their own errors.
And there is nothing they can do. Be false around Nico just because of the Oracle won't change things. They have to be genuine, and they have to really mean it.