WARNING: The following contains spoilers for Season 4, Episode 11 of Attack on Titan, "The Deceiver," now streaming on Crunchyroll, FUNimation, Amazon Prime and Hulu.
The final season of Attack on Titan has been filled with dark ethical dilemmas. As Eren's actions in Marley killed lots of innocent people, other characters, not to mention viewers, are missing the heroic Commander Erwin Smith right about now. After all, Erwin was the epitome of heroism and always strived to make the correct and moral decision when faced with obstacles.
His death in Season 3, Episode 18, "Midnight Sun," was a hard blow to the Scouts, especially when it was discovered that Armin was chosen to live instead of him. But judging where things are going in the anime, perhaps it's for the better that Erwin is gone now. He wouldn't have been able to handle the events in Paradis and Marley, and it seems unlikely he'd have stood behind Eren and the Scouts as they plunge down their violent path.
Erwin Smith was Attack on Titan's archetypal hero character. He was pure-hearted and fought for justice and peace, which made him an exemplary leader for the Survey Corps. He seemed like the natural choice to inherit the Colossal Titan, so it was a shocker when Levi ultimately chose Armin. With Erwin's outstanding leadership skills and years of experience, Armin seemed like the wrong choice. But would Erwin have been able to fully command and utilize the Colossal Titan in the same capacity as Armin?
It's difficult to imagine Erwin stomping through Marley as the Colossal Titan. Erwin wouldn't have gone along with Eren and Zeke's plan; he was too pure-hearted to ever sacrifice hundreds of innocent lives, even if they were Marleyans. Had Erwin been saved and given the Colossal Titan, he most likely would have wholeheartedly disagreed with using the Titan for any purpose. Levi made the correct choice in choosing Armin - with Armin's love for Eren as well as his quick, logical thinking, Armin can fully utilize the Colossal Titan if it means it's for his friends and for the survival of his people. Season 4 of Attack on Titan just isn't the landscape for real heroes -- the moral and ethical dilemmas would have potentially crushed Erwin.
However, Erwin's legacy still plays an important role in Season 4 - it keeps Levi's head on his shoulders and guides him through difficult choices. Because Levi held Erwin in such high regard, there's no doubt that his morals and presence loom over Levi as he conducts his missions with the Scouts. If no one else, Levi is the perfect person to carry out Erwin's way of thinking. He even still wears the classic green cape that no other member of the Survey Corps seems to have kept, keeping the reminder of Erwin close at hand.
Erwin's legacy of peace, thoughtfulness, and honesty is what Season 4 truly needs. Levi carrying it forward is entirely appropriate and may eventually set him at odds against his own regiment. But Ewin wouldn't have been able to be his true self in the world of Season 4. Being a real hero isn't possible when there are this many lives on the line and every choice can make or break an entire nation. Erwin was excellent at leading his regiment and always guiding them down the right path, but there doesn't seem to be one right path for the relations of Marley and Paradis. It's likely that Erwin would have struggled against his own morals and found it difficult to make choices he wholeheartedly believed in.
While Erwin's death was a devastating loss, it couldn't have happened at a better time for both himself and his regiment. Levi took his legacy and continuously carries it forward, trying to find a balance between mass warfare and justice. Erwin, looking down on his Scouts, would have a hard time reconciling their actions against the morals they've upheld for a century. Erwin most likely couldn't have stopped Eren from going down his dark path, but Levi carries on Erwin's legacy as he tries to deter Eren from his morally questionable actions.