Show became a failure for not having Lancelot

While it is a poor show, it's not necessarily for that reason. Lancelot is a relatively later addition to the Arthurian legend. Some versions make Gawain the greatest warrior/knight. Even earlier versions point to Kay, closely followed by Bedivere (not appearing in this show).

Lancelot would have been introduced in season 2, hence with the Round Table being built at the end, and the hint of another Champion, with Arthur saying no one will be able to fill Leontes's spot, which we know Lancelot most likely will. Buuuuut now we won't be able to see that.... Damn, if Starz continued with it, I bet season 2 would have be an enormous improvement, and I loved the first season.... Shame, so much they could do, yet they just up and cancel it.

Winter Is Coming.
A Dance With Dragons July 12th.

I've never seen an Arthurian adaption that starts with Lancelot in Arthur's court. Lancelot is a foreign knight who hears about Camelot and the round table, and decides that he needs to serve King Arthur because he is the best knight in the world.

Of course, some legends say Gawain is the best; however, we don't get into that right now.

I really liked Leontis, though. Sad about his death.

Geronimo

The presence or absence of Lancelot is irrelevant.

The show was a failure because they had a total casting failure with the King, and didnt managed to come up with believable, original, or otherwise convincing scripts and storylines.

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A gentleman will not insult me, and no man not a gentleman can insult me.

No OP, the show became a failure because it was terrible.