Thinking about it more, Ina and Gura are pretty much MIA, but they haven't graduated/affiliated still. Then again, I doubt either Ina nor Gura will stream all that much on their PL, so it makes sense to keep revenue from merch/royalty if they aren't going to do much with streaming anyways (unlike Ame and Fauna, who wants to be streaming).
Mumei on the other hand is AWOL due to school, and she hasn't graduated? I thought with all the coursework on top of Holo stuff, if anyone from promise will graduate first it will be her. But maybe she will ride on the mostly passive income from Holo since she can't stream all that much due to her school?
So yeah, IMHO the people most likely to stay by generation:
Myth
Calli - never graduate while she has the record deal, plus she's in Japan anyways. She also pretty much outstreams vast majority of HoloEN, so very unlikely
Kiara - don't watch her much, but clips of her shows she's pretty happy with Holo
Gura/Ina - they aren't streaming in their PL, so just do the bare minimum and still rake in passive income. May just be affiliate if they can't even make the bare minimum in the future, but I'm pretty sure Cover is bending over backwards in keeping them from officially graduating, especially Gura
Promise
IRyS - specifically said she will be in Holo for a LONG time, and she already lives in JP
Bae - she moved to JP because she specifically said she didn't like travelling so much from Aussie, so not her
Kronii - Still in Canada, but like Fauna her streaming is erratic at best and her idol power is just a tier smidge above Fauna's. Another likely candidate
Mumei - I frigging love Mooms, but I can feel it in my bones she will be next
Advent
Biboo - won't quit since she moved to JP specifically for holo
Fuwamoco - pretty sure same reason as Biboo, but I don't remember them specifically saying it. Still, they ain't leaving till they get their VN bday wish
Shiori - she was struggling so much as Natsumi Moe, I just can't see her leaving even if she is Fauna/Ame tier in terms of idol power
Nerissa - she has health issues with her throat and seems content to be living in Ravencroft home, so maybe her next year?