Agreed, why not use the dragon to fly Arthur to the island and keep him alive? I would have, after 5 seasons of watching, I expected Merlin to out himself as he did to Arthur and even if the series was ending, but why not at least have Arthur alive and taking care of his kingdom with his queen and Merlin at his side for an ending?
I was depressed at the ending even though Merlin and Arthur got to talking about his magic and about the help Arthur got along throughout the series from Merlin.
Although it's just a TV series, the writers did me no justice in Arthur dying, I did not expect that from the writers at all. I wanted a "happily ever after" instead, maybe I am the only one, but watching Merlin as an old man walk down a road in the present century, it did nothing for me, unless there is another series coming up.
I have read that the Colin Morgan, who played Merlin wanted out to pursue other acting roles, what I don't know, but that shouldn't have stopped the series - or - have it end so miserably, IMO.