For all the things this fandom refuses to believe and chalks up to John's lies, the thing that baffles me to see so many believe without question is the idea of Perfect Lyctorhood.
Guys. Guys, there is no Perfect Lyctorhood.
Or at best, if there hypothetically could be, it's nothing we've ever seen. Paul is the closest thing and I know a lot of you would not consider Paul perfect. John did not achieve Perfect Lyctorhood, and it wasn't even his idea to claim he did. A quarter of NtN extensively details that he didn't.
The old Lyctors didn't know what Alecto was. John definitely told them more than he would have liked to, because of course she doesn't lie and is too obviously inhuman to hide it fully. But if they knew everything, Mercy wouldn't doubt that Alecto ever had a genetic code; she would know she didn't, or that any genes she might've had were made from John's own blood and bone.
Because they didn't know what she actually was or what actually happened (foreshadowed too by Mercy's "if you had lied about anything else" lines, when actually he did), they drew the wrong conclusion. They assumed something different in his process allowed Alecto to persist. But we now know the truth is that Alecto was simply too big to consume. She didn't die because she was already limitless. This will never apply to another human. But he lets them believe their conclusion because he thinks it's better and easier to talk his way out of than them figuring out the real truth.
It does remain possible that Anastasia and Samael were genuinely on the cusp of that breakthrough, but I honestly doubt it. That was another conclusion drawn by the Lyctors as a follow-up to the previous wrong one, and when John answers, he visibly hesitates. It feels like he's once again going, "....Sssure, yes, let's go with that." I don't know what Samael and Anastasia WERE on the verge of. Maybe they would have become gestalt like Paul, and the possibility of just one dying was why Pal begged Cam "don't look back", and John was afraid of the power they'd achieve (could Paul have greater thalergy than a normal Lyctor?) and/or of just the others seeing a different process and getting mad at him.
AND/OR, ACTUALLY? Especially if their attempt was one of the earlier ones (around the middle rather than the end), but even if it wasn't: I think a Paul situation has a STRONG possibility of being exactly what happened. John's most outright lies are usually the ones other people tell that he just nods along with. When it's from himself, if it's not feigned incompetence, he usually goes for half-truths and misleading truths. He says Anastasia panicked halfway through and if he hadn't stepped in they would have both died. I think it's very possible that John panicked halfway through as he realized what they were doing, and that it's genuinely true they would have both died— in the same way Camilla and Palamedes both died, to create someone new.
And we know how much John hates change. How desperately John needs to keep his specific people close. What are the odds he was so afraid of losing both of them and being left with a new person he didn't know, couldn't predict, and couldn't easily control with them having a whole Lyctor's power and maybe more? Especially if Cyth and Loveday, Cassy and Nigella, Cyrus and Valancy, Ulysses and Titania, maybe even G1deon and Pyrrha— if any others hadn't undergone the process yet, and there was a chance they'd see Samastastia and decide that was the path they wanted too. If he thought this meant he might lose all his friends instead of only the less favored half.
Either way, though, based on everything we know, there is no simple soul swap that results in dual immortality. Even John and Alecto involve a fusion of megasoul. "You and she are one." (This is also likely how a seemingly real facet of John could talk to Harrow in Alecto's dream.) And we've seen through NtN, the soul longs for the body. The body longs for the soul. A body housing a different soul doesn't last long, even when those souls ARE semi connected. A body even temporarily renting space to a foreign soul is a massive strain, like Cam carrying Pal.
Lyctorhood inherently involves death and consumption and acting against nature. It is the indelible sin. It's possible that Grand Lysis avoids that sin by making it about mutual death, about giving instead of taking, but it's still bittersweet at best. I highly doubt we're going to see a perfect solution that fixes everything, at least via more necromancy, because that's not the kind of series this is. It's messy, beautiful in its flaws, embracing the understanding that life is change and things can never be exactly as they were, and can rarely be exactly what you want, and letting go and moving on are necessary parts of life eventually.
Don't misunderstand! I do think Gideon will either be resurrected (perhaps the last true one ever) or there will be another way for her and Harrow to happily be together. In Gideon's case, there was nothing natural about her death, and the decision to say "no" is a rejection of the system that led to it.
I just also think the odds of rewriting the laws of life and death entirely are more likely than Lyctorhood But With No Consequences. It always has consequences. There is no Perfect Lyctorhood, but there's something good on the horizon, whatever form it takes. After all...
"There are more worlds than this. Come with us. We are the love that is perfected by death, but even death will be no more. Death can also die. There's still time, Ianthe. Time for you and for Naberius Tern."