Yeah I hope they don't as well. I did read one FF where they did that and it was a good set up where Lancelot,Gwen and Arthur (with Merlin's permission as he was exiled) were going to have an arrangement where they got to be with the ones they wanted to be with whilst maintaining the monarchy (not sure what they'd do about the heir thing though as its a WiP but I can't really see that going over in the show. I also read a myth where there were 2 Gwen's one a maid and one a princess they were identical twins and they had the same name and the on;y difference was a crown birthmark on the princess, the maid was the one having the affair with Lancelot to set up the princess. I could see the show doing a two parter either bodyswap or doppelganger using that.

I think they are going to use what you said and having Arthur a great king based on merit rather than birth/blood as that is what fits in with the ideals of the show and to be fair it is a whole lot more interesting to watch that develop and to see how Arthur will essentially tear down rule and regulations that have been around for centuries (give or take) to create an admittedly modern society rather than keep the rules that have traditionally governed them.