What happened? (spoilers)

I came in here planning to ask if the series ever picked up in its five seasons (having lost interest shortly after starting season 2), but taking a look at all of the "The ending ruined my life!" threads gives me the impression that the writers decided to tread water for five years and then abruptly kill all of the characters and end the story on a dour note out of some petty resentment for the cast not wanting to stick around for season 6.

So my question is...why the hell didn't they do something with the story before season 6? There was so much potential in this series, but as I began to move through it became increasingly obvious that the writers were afraid to change up the formula or advance the story or character relationships in a meaningful way. And sure enough, all the way until the final season they seem to be writing the very same silly idiot plots where Arthur gets knocked out so Merlin can Merlin, or Merlin hides behind a stump and secretly magicks at some problems. Then everyone dies and it's the future and the audience can screw.

Why didn't this show ever go anywhere? Why did it stick the same boring formula and refuse to embrace the stuff of Arthurian legend that people actually wanted to see? I keep reading, "Season 6 would have been great!". That seems like a cop out, to me. Why couldn't the previous five have been great? You can't hold the same note for five seasons and then say "Well, I was going to change it up but you stopped me!".