I can understand the concept of a rookie wall in football - these players are used to playing 12 or so games against college level competition, not 20+ (when you include preseason) against NFL level competition. That's a big transition.
I can understand the concept of a rookie wall in baseball for pitchers - these guys are used to throwing maybe 100-150 innings against MiLB competition, not 150-200 innings against MLB competition. That, too, is a big transition.
But, outside of maybe catcher, I just don't see the concept of a rookie wall applying to baseball position players. So they play more games, and the pitching is better. But all they're doing is standing around in the field for a long time, chasing balls and throwing them, and then standing in the batter's box and swinging at pitches, than running around the bases. These are finely tuned athletes, so the extra physical activity that comes from performing the above activities in more games shouldn't really be an issue. Mentally, yes, it can be draining, but that's part of the game for everyone. Physically, it should be easier for these guys than for seasoned vets with a lot more mileage on their bodies.