If I remember my lore correctly, magic existed in ancient times before mankind became the dominant species on the planet, and then it came back after the cataclysm that destroyed human civilization.
I can't say I've ever been that fond of the "There used to be magic but it conveniently vanished before modern times" trope. The best handling I've seen of it was in Larry Niven's
The Magic Goes Away, which took a "logical fantasy" approach and treated the source of magic -- mana -- as a natural resource, a finite physical property of the world that could be depleted and that got used up. I think it was as much an allegory for the '70s oil shortages and environmentalist concerns over non-renewable resources as a logical approach to codifying magic. But what was cool about it was that the stories didn't just use the impermanence of magic as a throwaway excuse for why the past was different from the present -- rather, the consequences of the depletable nature of magic were themselves a driving force in the stories. For instance, I remember that one wizard character had a spell that would just sort of run exponentially until it used up all the mana in the local area, thereby cancelling out any opponent's magic. And the larger "energy crisis" of magic being on the verge of extinction drove a lot of the plotting.
Why is it an annoyance that Davies is the only one with an accent? Why should people living in the Pacific Northwest two thousand years from now have to speak with a non-North American accent? (Yes, I know accents in two thousand years will not likely sound anything like our modern accents do. That's not the point.)
Also, it's not like this is anywhere near the first high-fantasy TV series to use contemporary language and American accents. See also
Hercules, Xena, and
Legends of the Seeker (all of which were also filmed in New Zealand, by the way).
Speaking of which, Rhys-Davies isn't the only one using a non-American accent. Manu Bennett seems to be using his normal New Zealand accent, IIRC. Hmm, given that they were contemporaries back in the time of the war, could it be a generational thing?
I think the point is: It's implausible for characters from the same culture, same geographic region, same time period, and same family to have different accents from each other. It's possible but not very likely. (Regardless of which accents we're talking about.)
But it does happen rather a lot in SF and fantasy. Like Roy Dotrice playing Hercules's father in the later seasons of the TV series. Or David Warner's Gul Madred having an English accent while most of his fellow Cardassians did not. Or Sean Connery as Harrison Ford's dad.