The game is considerably different than it was 35 years ago. The coaching staff is, what, double the size now? Managers used to run drills on the field, too, but they don't really do that anymore, either.
The coaching staff is far more compartmentalized today than it was years ago, probably due to the scope of analytics and the increased burden of responsibility placed on any single coach (ie. having to read, understand, and then coach based on the data, which just didn't exist in the game 35 years ago).
Wes Johnson was hired because he understood all that data and what to do with it, while Rocco Baldelli probably couldn't throw a breaking ball to save his life.