It’s exactly like trying to explain Frank Miller’s Sin City concept to mass audiences. It’s just never going to happen, no matter how well executed. They’re trying to sell the wrong concept to the wrong audience. Even Eastwood hasn’t been able to reliably pull off the Man with No Name persona in recent decades. His most distinctive hits in a starring role have been Unforgiven (the all-but “last Western”) and Gran Torino (Crotchety Old Man Times A Thousand).
Call Elba “the gunslinger” and say it’s just a name, and you’re missing the huge disconnect, because you’re too close to the material. You know what he’s supposed to be. But everyone currently failing to make heads or tails of the concept is just not getting it. They’re confused in the same way you think it looks like generic YA material.
Roland was a warrior (as the Warren Zevon song goes), and not just a “gunslinger.” But they chose to define him as a gunslinger, because they had no confidence in the material.