alex has noticeably improved his pitch selection this year, and billy has done the opposite. it's weird.
I don't disagree that Billy's physical makeup may ultimately cause him to drop off more quickly, but I don't think that's what's happening here, just yet. From what I've read, Billy's big problem is he's swinging at worse pitches this year, as you note.
Also, if Billy chose to adopt Jose Bautista's approach, then he could be a 30+ HR guy, I think. He has chosen to hit for average and gap power instead. It will be interesting to see (if he straightens out his current issues) whether he ever chooses to go power over average. It wouldn't necessarily be the worst decision for someone with his, uh, speed/physique. Wouldn't surprise me to see him go this way if he leaves the Royals (and the perceived/actual HR-killing confines of the K).