Where did this theory about someone dying even come from? We have Calypso on our team. Remember how she revived Orion after he was shot in the back and lost a lot of blood.
Some other people answered you, but there's also foreshadowing in 0.9 with Luna and the MC's conversation about what they would do if the other died (in the context of the movie they had watched). The MC says that if Luna died he'd lose his mind instantly, but Luna says that if the MC died she'd "join a necromancer cult" so that she could revive the MC, since surely in a world already so fantastical, there must be "at least one witch specializing in resurrections". There is also the MC saying that we'd never bring one of the girls with us into Eternum with our powers again, since it's simply too dangerous and that they could really die that way, but now going into Eternum with the implant is about the same risk, if not even riskier since you need to get to an exit portal, when with our powers we can act as the exit portal for them.
Thinking more specifically about Luna's phrasing of "join a necromancer cult", I'm replaying the game, and I completely forgot that in our initial dream where we see the "Lady" keep asking "Will you help us?", we're set in a church, a church that we later see at Warthogs with Luna and Penny. Luna tells us that there used to be a cult there, and that cults are allowed on Eternum as long as they aren't otherwise breaking the code of ethics, but this cult ended up forbidden and the praetorians turned the place inside out.
It's heavily implied that this cult was the Syndicate, especially adding the context of one of the praetorians asking "A member of the Syndicate?" to the praetorian that excommunicated Benjamin. From what we know about the Syndicate, they definitely seem to fit what one would call a "necromancer cult", and we have a nightmare when sleeping in class of waking up in a morgue, seeming as if we were a corpse that just woke up. There's a seeming-corpse next to us, that then gets up and is identified as "Lady" and again asks "Will you help us?" and then asks us if we would press a button if it meant there was a 1 in 7 chance of saving the love of our life, who is about to die, but a 6 in 7 chance of the button killing us.
This all would maybe seem to be foreshadowing our own death, but I feel like at this point we've already nearly died too much for that to really hit as hard, and combined with the stuff other people mentioned, and the cliffhanger of 0.9 clearly wanting us afraid for the girls' lives, I really feel like it's leading up to the death of one of the girls. Which in my mind would most likely be Annie, but it could well be a different girl too. It could maybe be that we would die with the girl together, getting that 6 in 7 chance of the button where we both die, and we would both become NPCs I guess. I'd worry perhaps though that the MC turning into an NPC or otherwise dying would restrict the plot too much, even if it would be exciting to see the rest of our girls show how competent they are even when we're not there.
(As an aside, that is something that I found kind of lacking in 0.9, the girls trying to track us down and not really getting anywhere. They were by no means incompetent, but I wish they would've gotten further. Nova was onto
something, but clearly still very off the mark, though I suspect she's probably right that Hyril'ar plays some crucial role with the Eternum system. I didn't even need them to get close to finding Hyril'ar, they still could have advanced the plot/our known information another way, like maybe by tracking down Thanatos and finding what might've happened with him and where he went off to. But it's fine, it probably needed to be the way we saw because there's only so much narrative space.)
But beyond any foreshadowing, I do think that one of the girls becoming an NPC would make a lot of narrative sense. It would conjoin our emotional drive of caring about the girls with the narrative drive of freeing the NPCs. The MC and the girls do already care about the NPCs, and by now that plays a nominal role in our goal of acquiring the gems of doom, but that care for the NPCs ultimately remains secondary to our care for the girls and exploring our relationships and all that. But if that care for the girls becomes as well a care for the NPCs, then I think that would really drive the plot forward and really cement the transition into the endgame of the story.