Bertholdt's Colossal Titan also fell, and with a single dose of transformation serum remaining, either Armin or Erwin could be saved via a Bertholdt brunch. Intense debate ensued, but Levi ultimately decided to champion youth, choosing Armin's intellect and importance to the future over Erwin's leadership skills and unyielding determination.
Levi's decision to make Armin the Colossal Titan instead of Erwin was not universally popular, and many characters have questioned whether anime's best bowl-cut made the right call - not least Armin himself. Plenty of occasions since that fateful battle called for Erwin's authoritative hand, leaving Survey Corps officers to lament having a shy, floppy-haired book geek instead of their fearless leader. When Attack on Titan season 4, part 2 commences, Paradis Island's military has been usurped by Eren's Jaegerists, while Armin and his friends rot inside a jail cell. Eren, meanwhile, runs rampant outside, fending off invasion from a vengeful Marleyan military... and it's here that Armin finally justifies Levi's decision to save him.
At this point in Attack on Titan season 4's narrative, Mikasa and Armin still don't fully understand Eren's goals and motivations. They know he abandoned any semblance of morality due to the Liberio incident, and they endured a vicious diatribe in Attack on Titan season 4, part 1, where Eren told Mikasa he hated her. Nevertheless, many questions remain, and Armin's Sherlock Holmes-esque deduction skills duly fill in the gaps of Eren's masterplan. In "Judgement," Armin ionately tells Mikasa that Eren's Ackerman "revelation" was about as trustworthy as Sasha around a sack of potatoes. Armin believes Eren fabricated the story about Mikasa's bloodline to prevent her getting caught up in his scheme. Detective Arlert then confidently declares Eren Jaeger would never agree to Zeke's euthanization plan, even though the likes of Jean and Connie all have doubts. Armin doesn't stop there with the rapid-fire tactical calculations. The Founding Titan power requires Eren to cooperate with half-brother Zeke - a Titan of royal blood. According to Armin, however, it'll be Eren who decides how that power is used. Eren could, therefore, be exploiting Zeke simply to unlock the Founding Titan's abilities.
Only Armin could've made these vital strategic insights at such a tense and critical juncture in Attack on Titan's endgame. As leaders, Erwin, Hange and Levi are/were all tactically astute, but they lacked any real personal insight into Eren's psyche. Mikasa, Jean and Connie all know Eren on a deeper level, but are missing the intellectual nous to piece together their friend's secret plan. Armed with a brilliant mind and an intimate knowledge of Eren's mentality, no one but Armin Arlert could've possibly figured out Eren's intentions in Attack on Titan's final arc. The military (or what's left of it) can now take an informed course of action, rather than continuing to believe Eren's goal is rendering the entire Eldian population infertile.
When Levi was standing over Armin and Erwin deliberating which should be consigned to death, his decision wasn't based on who would prove most useful if Eren Jaeger ever broke bad. But something inside Levi decided Armin was the greater asset, and Attack on Titan season 4's "Judgement" finds Armin dropping analytical bombshells that could change the very course of Eldia's war - a contribution Erwin Smith could never have made.