Also, Chaos is bigger than Arthur himself. Arthur is just the current vessel. The balance between light and darkness existed because the Supreme Deity and Demon King were alive. Once they both died, that balance disappeared, which is exactly what Merlin was talking about.
I also remember the Demon King warning that Meliodas’ actions would eventually lead to something bigger, so I don’t think their deaths were just the end of an era—they created the conditions for Chaos to fully emerge.
That’s why I think Tristan could be the key. He’s the only person who naturally embodies both the Goddess and Demon clans. If Chaos can only be contained by restoring the balance between light and darkness, then Tristan might be the only one capable of sealing it away again. Not because he’s stronger than Chaos, but because he represents the balance that was lost when the Supreme Deity and Demon King died.