The sales of NA aren't falling because of T'Challa. He's only one person in an ensemble cast. Even if Hickman revealed his full KoD powers, that would be enough to stop falling sales. That's just the nature of the business. The title got derailed by Infinity(not Hickman's fault) which is taking the story that much longer to tell. Really the problem is the patience of readers. Are readers willing to followthe story through to the end? Do they need a quick fix to get them through? We are a fast food, tweeting, selfie cultural that doesn't have time to sit and read. We want what we want and we want it now.
I'll agree that there needs to be more action and some feats shown by everyone and not just BP. We're seeing some of it now. It now depends on whether or not most fans have the patience to see it through to the end.