I know we all laugh about this line because from our perspective Arthur is blind and stupid for not noticing sooner, but I think it actually says something about Arthurâs view on magic.
At this point, heâs not exactly anti-magic the way Uther was (he never really is). He went to the Dolma for help, and acknowledges that the sorcerer helped them win the battle.
However, he is definitely still prejudice against it. But whatâs one of the biggest reasons that he doesnât trust it? Morgana.
Arthur watched someone he considered family go from the caring and kind Morgana he grew up with, to the brutal and cruel person she is now. Itâs when she betrays them that he learns about her magic.
As an outsider, all he knows is that Morgana is using magic and betrayed all of them because of it.
In his mind, he is convinced that magic corrupts people. Almost every magic user heâs ever met has either: tried to kill him, tried to kill his father, or tried to seize power at the expense of others. The only true exception is the Dolma, because he blames Dragoon for his fatherâs death for a long time.
Morgana is the biggest influence on his view of magic; he watched her fall into darkness because of it. He genuinely sees magic as a corruptive force that turns even the nicest people, like Morgana to him, into monsters.
Arthur has had him by his side for years. In his mind, if Merlin had been using magic that whole time, he wouldâve been corrupted by it. He canât comprehend Merlin having magic because he associates it with greed, corruption, and cruelty, and Merlin is the exact opposite.
A lot of fanfics use this concept when writing their own versions of the reveal. To some, it can come across as sappy or leaning too much into the shipping territory, but I wouldnât be surprised if it was accurate to some extent.
During the first real conversation after the reveal, Arthur asks him a very important question: âWhy are you still behaving like a servant?â
What makes this important is the insight into Arthurâs thoughts. He canât fathom that a person with magic would serve someone, willingly. In his mind, as soon as Merlin âhas magicâ, he is no longer a servant.
Why would he be? He could topple empires single-handedly. He can create and take life with a few muttered words. He can conjure the elements and manipulate objects with just a thought. Why on earth would a man with that power choose to be a servant?
Arthur thinks that itâs impossible, that no one would choose that. But here Merlin is, taking care of him the same way he always does, even more so given how injured he is. He canât believe Merlin would choose to serve him rather than pursue power.
I think thatâs the biggest reason he says: âI would knowâ. Arthur thinks he would, because he assumes Merlin would change somehow.