I'm not sure... Even disregarding Armin being more of a main character, I think that the story has been strongly pushing for Armin to be a sort of successor to Erwin- with his potentially last act, he definitely proved that his current self is not lacking in bravery or intelligence.
Plus there's that fairly common notion that it's the job of the younger generation to carry the mantle. I really feel like Armin's gonna be the one who will be injected.