I wonder if to some degree it isn't modern baseball and Cubs player selection?
1. I think high-end guys like Almora, Bryant, Russell, they'd all gotten lots of good coaching prior to joining the Cubs? Maybe there isn't as much additional intellectual information to learn for guys like that as your average guy their age might have had decades ago in the pre-saber and pre-internet era? Maybe they had nothing left to learn by the time they came up?
2. Same for nutrition and stuff, for physical maturation? Almora had some, yes, but Bryant, Schwarber, Russell, are they any more physically mature and ready than when they were rookies? Over baseball history that maybe isn't routinely true for young-20's guys. But maybe there was nothing left to develop physically by the time they came up?
3. The Cubs have put a priority on smart, motivated, thinking players with their top-8 draft picks. Again, maybe they were so naturally smart and inquisitive and motivated that they'd kind of already gained whatever head knowledge they could gain by the time they came up? Maybe smart guys like them don't leave as much left to learn by the time they are rookies?
Saber history I believe indicates that young hitters tend to get better and improve their power production after their rookie year. I'm wondering whether for Bryant/Schwarber/Russell/Almora/Happ types, they were already at their physical, mental, and psychological/confidence ceilings when they came up and started fast? And have had nowhere to go but ideally plateau or else decline subsequently?
None of those guys have seemed to adjust in any significant positive way.