If I may be weird and petty for a moment, I don't get AUs where Martin is the Archivist when Melanie and Sasha are right there?? I mean I get why: roleswaps are popular AUs and martin is a popular character. And maybe its just my specific headcanons re: archivist requirements but I feel like the other two would be so much better?

Yes, Martin reads statements and yes, he gets marked, but I really don't think those are the Only Requirements to do the big ritual? Cause at that point Elias could've chosen any web-marked dude off the street, especially if they have the mark of another entity as well already.

Melanie and Sasha shared several important traits with Jon, most notably a burning curiosity, a willingness to put themselves in trouble for information, and (in Melanie's case (and Sasha's too, arguably) an ability to antagonize extremely dangerous fear avatars. Sasha's whole statement is her putting herself in a dangerous situation without telling anyone or acquiring any help (remind you of any main characters?) and running face first into supernatural dangers (sometimes to protect her friends, sometimes out of curiosity). Useful for collecting marks, but also useful for collecting information and developing her own powers as the Archivist. Considering her seemingly unrepentant willingness to look through her friends' and coworkers' personal files via accessing their accounts, I'd argue that her curiosity and boundary crossing could easily get stronger when investigating supernatural entities. Obviously, this never happened because of how early on she died, but I think you could make a strong case for Sasha's strong curiosity making some avatars incredibly mad. Not to mention how incredibly Beholding it is of her. None of this is criticism btw, I think Sasha should have access to whatever personal info she wants and I love it when characters are so obviously Eye-aligned. I know "Sasha wasn't promoted because she'd solve all the problems and find a way to kill Jonah" is also a popular headcanon but please think for a MOMENT about her appearances in podcast. There weren't many, but there's a very obvious through-line in almost all of them. She's curious, she takes risks in the face of the supernatural, she wants to Know. It's literally what caused the circumstances of her statement. Those traits are arguably the ones that got her KILLED. I'm sorry this went off on a tangent. I just love Sasha very much and I'm constantly filled with Thoughts of Sasha.

Melanie also has curiosity about the supernatural AND a much more obviously antagonistic personality!! Her first appearance is being a total asshole to Jon, mocking him, the place he works at, the equipment they use, and what they do. But she also dedicated basically her entire career into uncovering the supernatural? She ran a Youtube channel about it, and when she discovered that everyone was sticking to the same "safe" sites, she started looking off the beaten path for the real stuff. And she found it! She went looking for ghosts of war and violence and even without access to statements, probably working alone and sifting through bunk on the internet, she found them! She got stabbed by a ghost and instead of trying to avoid ghosts in the future, she decided to look for more violent ghosts. The slaughter tendencies might be a drawback for her becoming the Archivist, yes, but by that point Jon had already survived multiple marks and was developing his powers. If he failed by that point, Elias might've wanted an Archivist who'd be better able to defend themselves. By S4 Melanie was probably far enough gone to the Slaughter (and hated the Eye enough) that she was probably off the table as a candidate. By that point, however, Jon was expected to fully Become and only needed a few more marks.

I understand that Sasha was approved for a transfer to the Archives because Elias let Jon choose who take with him and Jon chose Sasha. And Melanie was almost certainly chosen to keep Jon in close proximity of a Slaughter-aligned person who'd be likely to mark him but unlikely to kill him. But I think that they served a secondary purpose of being backup Archivists in case something happened to Jon.

Obviously this is more headcanon than theory (not to mention it relies on other headcanons to fully stand) but I'm very fond of it. I guess I think that, even if Elias wasn't really looking for another Archivist when Jon seems like the perfect candidate in every way (and I'm not saying this JUST because I love him), I think that if something did happen to Jon, they'd be Elias' next candidates.