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I think the reason non of the other ghosts have been sucked off yet is because they don’t want to.
Mary has said before that she wanted to be sucked off:
In The Thomas Thorne Affair(S2E4) she says “I used to dream of the day I would be sucked off’.
In Gone Gone(S4E4) she calls Robin a “lucky beggar’ when they think he got sucked off.
Whereas the other ghosts never express any want to move on:
In The Hardest Word(S4E3) the review(that Mary didn’t type) they say they never wanted to leave.
In Carpe Diem(S5E5) they’re all rather panicked at the thought that one of them may be sucked off.
So while I do think the ‘unfinished business’ trope does play a part in who gets sucked off (i.g. Mary had to talk about the trial), but those who want to stay will.
What if Robins sucked off prediction math was correct and the main ghosts just didn’t notice?
One of the plague ghosts going missing would probably go unnoticed by anyone but the other plague ghosts