There is literally nothing that can justify the fact that Arthur didn’t find out about Merlin’s magic until he was dying. Nothing. Not as a long term writing choice, at least. TV shows, while they’re meant to keep us on our toes are not meant to frustrate us with five years worth of miscommunication (that’s even longer in canon). TV is too sophisticated for that these days. Merlin and Arthur had such a brilliant chemistry but the story of their friendship was tainted by this lie. What other TV duo had such a division that was kept secret?
People write so much fix it fanfiction about this one thing simply because it was so frustrating. What would have happened if Arthur had found out? Well, obviously a better story according to the hundreds of possibilities that are more emotionally fulfilling whilst being just as, if not more, engaging as the canon show.
When I watch other TV shows based on the same premise: two opposites united by destiny, it’s so much more satisfying because their relationship is not based on a humongous lie. BBC’s Merlin could have gone to such a fantastic place if only it had just let go of this gimmick of “Merlin’s magic is secret! shhh!” It worked for a series but then should have been dropped.
I can remember the first time I was starting to suspect Arthur wasn’t actually going to find out. It was the first episode of the second series and I remember thinking “right, we’ve had the first series to establish the situation and relationship, now we’re on the second series and Arthur is definitely gonna find out!” and I was expecting it to go bam and launch us into the next phase of the story but it never did. I was still hanging on, waiting for it to happen until series four. Then, when we got to s5 I finally went: “he’s not going to find out until the final episode when he dies is he?” and guess what happened.
It is one of those things in television that I will never be okay with.