Honestly, I think people underestimate Tristan's role just because he’s the kid of the series. His existence is basically a giant plot coupon, but not in a lazy way. The whole peace between demons and goddesses hangs on him, right? That’s not just backstory—it constantly forces the Sins, especially Meliodas and Elizabeth, into this impossible position of protecting the symbol of peace while every faction wants a piece of him. He’s the reason the final arc even has stakes beyond 'beat the big bad.' Without Tristan as a living legacy, Meliodas’s entire arc about overcoming his demon king destiny feels less grounded. The kid is a walking consequence.
Also, the generational thing. Seeing Meliodas, who was always this reckless, emotionally stunted powerhouse, suddenly having to be a father—or at least figure out what that means—adds a layer the earlier seasons never touched. It shifts his character from the perpetually suffering immortal to someone building a future. That’s huge for the story’s emotional resolution, even if Tristan himself isn’t on screen much.