WARNING: The following contains spoilers for Attack on Titan Season 4, Episode 17, "Judgment," now streaming on Funimation, Hulu and Crunchyroll.
One can only wonder what the late Erwin Smith would think of the current state of affairs in Attack on Titan. Whether he would agree with what Eren Jaeger has done (i.e. starting a war with the whole world) is a moot point considering his spiritual heir, Armin Arlert, has decided that the precious few remaining members of the Survey Corps are going to help Eren.
Finally stepping into the leadership role that's been his destiny since Levi turned him into a Titan, Armin remains objective about the situation, even giving Onyankopon an opportunity to explain himself when he very much looked like yet another traitor.
Armin Is Finally the Leader He Was Always Meant to Be
Ever since he was turned into a Titan instead of Erwin and had Bertholdt fed to him, Armin has felt the weight of carrying on Erwin's legacy. It has been heavy at times, as he did not even agree with the decision once he'd heard what happened in the immediate aftermath. Yet Armin has proved his worth as a leader in Attack on Titan, steadfast in his belief that the least bloody path to peace should always be the plan.
Armin does not wear rose-colored lenses when it comes it his best friend, but it's to his credit as a leader that he stops to consider what he doesn't know about Eren's actions, instead of only acting on what he does know. Armin posits that they don't actually have a reason to believe Eren intends to carry out the Euthanasia Plan. He knows Eren remains their best chance at surviving Marley's assault, and convinces his friends by reminding them Eren's whole deal is freedom for Eldians. After all, forcefully sterilizing his own race couldn't be more counterintuitive to his ambitions.
Is Onyankopon a Jaegerist?
Onyankopon has been working with Yelena since the Eldian Sympathizers from Marley first arrived on Paradis Island to help the Eldians catch up to the modern world. Considering most of those very same "sympathizers" turned out to be Zeke's acolytes who overthrew Paradis' military and named themselves Jaegerists, the question now is how much Onyankopon really knew about the plan. While he insisted he didn't know about Zeke's spinal fluid in the wine, if there's one thing Attack on Titan has proved throughout its history, it's that a fresh betrayal is usually just around the corner.
Eren keeping control of the Founding Titan is in everyone's best interest except Marley's, so Onyankopon freeing Armin and the others doesn't prove he's not a Jaegerist. When Connie would've torn Onyankopon to pieces from suspicion alone, Armin's cooler head prevailed. Onyankopon's plea in explaining himself was convincing, arguing there's no reason he would've dedicated so much of his life to helping Paradis Island just for Eren and Zeke to wipe out its population with the Euthanasia Plan. While Onyankopon appears to be truly on their side, it wouldn't be the first (or even the fourth) knife in the back these Survey Corps members have endured.
What Happens Now that Onyankopon Has Freed Armin & the Others?
Eren's entire plan to fight Marley apparently began and ended with Titan punching, which has gotten him nowhere. He and the Jaegerists are losing, and badly. If only a prodigal genius with blonde hair and an unrivaled strategic mind had just been freed from his prison cell.
Once Armin gets eyes on the battlefield, he should no doubt get right to work on a plan to keep Eren alive and in control of the Founding Titan. Pieck and Magath are currently the biggest threat with their long-range cannon. If Armin figures out how to remove them from the equation, Eren's War Hammer powers should allow him to resume his fight against Reiner and Galliard -- which he was winning before Magath nearly blew his entire head off.
It's unclear how Yelena might feel about Onyankopon leaving her side to go free her prisoners, but given how distraught she is over Eren's struggles, she'll probably welcome anything that may help save him. If there even still are Jaegerists after this battle, then Armin and the others officially have their man on the inside in Onyankopon. The thing about a war is that eventually someone is going to win, and as the de facto leader of the Survey Corps' tattered remains, Armin will be a vital piece of Attack on Titan's puzzle going forward -- regardless of who emerges victorious.