I think the fact that Arthur now listens to Merlin and actually takes his advice is a quantum leap in their relationship. He asked Merlin "do I allow sorcery back into my kingdom?" It was a pivotal moment. Merlin got it wrong, but for pure reasons. If they did have the courage of their convictions and kill Arthur at the end of this series it would have been too soon according to most legends. Which is why, I suppose, they decided to make more. However the doom-laden atmos is first class. Something bad is going down.