How do you know that's what the focus of The Defenders will be? I mean, we can probably reasonably assume that that's what the focus of Daredevil and Luke Cage's shows will be, but it is four separate shows building up to a mini series, not just one show. Jessica Jones can go to really weird places sometimes. And one of the shows is about Iron Fist, who was raised in an alternate dimension and got his powers from plunging his fists into the still beating heart of a dragon he had just killed by punching it to death, so we know that at least one of those shows is going to have strong elements of mysticism woven throughout it. There's no reason to assume that the plot of the Defenders mini will just be those four characters teaming up to beat up The Kingpin. Daredevil can deal with gangsters in his own show but then also be called to deal with a slightly more supernatural threat in the Defenders mini by Dr. Strange. It can be done. It wouldn't be that weird.