Can we talk about how significant this moment is for a second?
This is the guy that Erwin pulled off the street, who agreed just so he could kill Erwin, who stayed not out of loyalty to king or country or legion but to Erwin himself. So when Erwin was for all intents and purposes on his deathbed, it was entirely in the realm of possibility for Levi to wait for an outcome while perched on the ready to cut all ties and go. Instead, he carries on with his responsibilities and plans for the future.
Levi is not dumb; he wouldn’t for a second entertain the notion that Erwin would survive just because he “believed” hard enough or continued on like he would make it. No, this is Levi’s honest to god decision to stick with the Scouting Legion, and if you don’t think that gesture carries weight, take another good hard look at Erwin’s reaction.