The real implications of Arthur never knowing
One of the funniest/saddest/most interesting things about the Merthur relationship is that Arthur has no idea how unequal it is. Like it’s easy to get mad at him because we see all that Merlin’s done and sacrificed, but Arthur doesn’t. From his point of view he and Merlin have a fairly equitable arrangement going.
Sure, Arthur sometimes abuses his position of power in petty ways, and that’s not great, but he also lets Merlin get away with pretty much whatever. He lets Merlin talk back to him both in private and in front of people, which is a huge deal. Arthur is the king/crown prince, and he lets his servant disrespect him in public. He lets Merlin disappear to the tavern, sometimes for days, without any real punishment other than some yelling and some unpleasant chores he would have had to do anyway. From Arthur’s point of view, Merlin puts up with his bad behavior and in return, Arthur puts up with Merlin’s.
Arthur knows that Merlin has protected him and remained loyal to him far past anything that could be expected of a servant, but Arthur has also protected Merlin past what could be expected from an employer. He’s committed treason for Merlin, risked his life for Merlin, risked war to plead for Merlin’s life. Again, Arthur doesn’t know the lengths that Merlin has had to go to, doesn’t know how much he’s given and sacrificed without recognition or reward. He doesn’t know that Merlin is risking himself every day that he stays in Camelot. He only knows that they’re friends (even if he wouldn’t always admit it) so of course they’ll protect each other when they can.
Arthur also doesn’t know that Merlin can’t leave. He doesn’t know that Merlin is basically trapped by destiny and under a huge amount of pressure to take anything Arthur dishes out at him. Servants aren’t slaves. They can quit. Most of them don’t really have many other options but Arthur has every reason to believe that Merlin does. Merlin could go work for Gaius full time, or just go home to Ealdor if he was ever really unhappy. Except no, he can’t, because then he’d be failing all of Albion and also Arthur would die.
Merlin having magic and being Arthur’s destined protector changes everything about their friendship, makes it distinctly one sided and unfair, and makes every less than fantastic thing Arthur has ever done or said to Merlin about a million times worse and more abusive than Arthur ever intended. And he never gets a chance to make it right, to see his best friend as he really is and do something about it because the stupid writers never let him learn the stupid truth until he was dying.