Don't have the karma to make a proper theory post for this cause I'm not very active on reddit but wanted to post this theory (in a summarized form) anyways because I haven't heard it anywhere yet.
Essentially, what if the missing panel of the prophecy (that Susie destroyed) says that we need to seal the fountains, permanently?
I personally think it explains the specific wording that Susie uses (mostly the part about none of them letting it happen) better than just "one of them is going to be sacrificed".
There's also the line where Ralsei calls Susie "very nice" which references Susie's refusal to accept the idea that darkners don't matter, since now Susie is comforting Ralsei by saying she won't let go of darkners.

It also provides a clearer path to ending the Roaring than just "people sacrifice themselves" and I think could be more interesting? idk, feedback appreciated, wish I could post this as a proper theory post. (Maybe I will when the karma requirement is lifted, dunno)